20 Shorts to Explain Dark Matter | Shorts Supercut

[00:00:00]Dark matter is one of the biggest mysteries in the universe, but it isn't a single thing. Like, it's not even a single question. This is dark matter explained, episode one. What even is dark matter? Now, you guys, simply put, dark matter is like the name we give to a problem. That's all it is. When astronomers study the universe, gravity consistently behaves as if there is more mass present than we can account for with the matter that we can see. Now, that mismatch shows up in a number of completely different places. It's in how galaxies rotate. It's in how galaxy clusters move, in how light bends as it travels through space, and even in the structure of the early universe. From all of the observations that we have made, the total amount of matter that we can actually see is about 15% while 85% of it is transparent to us.

[00:00:58]And that's an enormous difference. The important point here is that this dark matter didn't start out as like a theory. It wasn't an idea or a particle or just, you know, some guess. It started as a description. It's a temporary label that we give to the fact that our observations don't fully line up with the calculations that we can make. Now, what the missing ingredient actually is remains a very open question. It could be a new kind of particle that barely interacts with ordinary matter. It could also be that gravity behaves slightly differently on a very large scale. It could be something entirely new that we just haven't put together yet. In this series, I want us to work through where the dark matter problem came from, what the evidence actually says so far, which ideas have and can be tested, and why after nearly a century, we still don't have an answer.

[00:01:59]It's going to be a long one, you guys, but I really hope you stick with me, cuz this is a bit mindmelty.

[00:02:06]Now, before we can talk about what dark matter might be, we need to talk about something like just a bit more basic.

[00:02:13]How do we decide that something exists at all, especially when you can't see it? But that's for the next episode.

[00:02:22]Stay nerdy. There are things passing through your body right now that you will never notice. Now, that doesn't mean that they're not real. This is Dark Matter Explained, episode two. How do we know something exists? Everything we know about the world comes from interaction.

[00:02:42]We see things because light interacts with our eyes. We feel things because atoms push back on each other. And we hear things because air molecules interact with our ears. Interaction is how information from the world around us gets to us. Now, physics gathers information in the same way. It just isn't limited to the interactions that our human senses evolved to notice.

[00:03:10]Nutrinos are a really good example of this. Huge numbers of nutrinos pass straight through the earth and through our bodies all of the time. Most of the time, they don't interact with anything at all. But every now and then one will interact inside a specially built detector in a very very specific way.

[00:03:32]Now we don't see the nutrino itself. We see the result of the interaction and that's enough to know that it exists.

[00:03:40]Astronomy is built almost entirely on this idea. Most of the interactions that we use in astronomy involve light of some form across the electromagnetic spectrum. By studying the light that we collect from like stars and galaxies, we can work out a lot of information like what they're made of, um, how they move, how far away they are, and even how hot they are. But light isn't the only interaction available to us. Gravity is another. Now, gravity, it doesn't depend on whether something shines or not. You see, if something has mass, then it will affect how other things move, including how light travels through space. And this is where dark matter comes into our picture. Now, we're not saying that dark matter exists. We haven't seen it. What we do, though, is we infer its existence because when we look out into the universe, there are signs that objects are experiencing more gravity. It's as if there's more mass there than we can see.

[00:04:49]But how do we even know how much mass there is out in the universe? Well, that's for the next episode. Stay nerdy.

[00:04:58]How do you weigh a galaxy? Like, we can't just put it on a scale, but somehow we do manage to work out how much mass they have. We just need motion and gravity. This is dark matter explained, episode 3. How do you weigh a galaxy? In our normal everyday situations, weighing something is pretty simple. We just put it on a scale.

[00:05:18]Gravity does the work for us. Now, in astronomy though, that option doesn't exist. We can't touch galaxies. We can't isolate them completely. So, instead of weighing them directly, we infer their mass from their motion. Now, this idea is older than modern astronomy. If something is orbiting or being held together by gravity, then its motion tells you how much mass must be there to make that motion possible. And this logic works incredibly well. We use it to measure the mass of the Earth from satellite orbits, the mass of the sun from planetary motion, and the mass of stars from binary systems. The key point is that gravity doesn't care like what something is made of. It responds only to how much of it is there. So when we look at a system say like a planet orbiting a star or a star orbiting the center of a galaxy, the speed of that motion encodes the total mass that's involved in the system.

[00:06:27]If the planet moves faster than gravity allows, then the object would just fly off outwards away from the system. If it moves slower, then it would just collapse inward towards the center of the system. As long as gravity behaves the way that we understand it to, then motion acts like a readout of the mass.

[00:06:50]And for most of astronomy, this method works extremely well and it is really consistent. It agrees with independent measurements whenever we can cross-check it. Which is why it was taken really seriously when at much larger scales it started to give answers that didn't make sense. About a hundred years ago, astronomers started applying this same method to galaxy clusters and the numbers didn't add up. And not like by a little, but by a lot. But that's for the next episode. Stay nerdy. The universe is missing some mass. In the 1930s, an astronomer looked at a cluster of galaxies and realized they should already have flown apart, but they hadn't. Why? This is Dark Matter Explained, episode 4, the first missing mass problem. Now, by the early 20th century, astronomers were getting reasonably good at applying gravity to large systems, things like stars, binary galaxy or binary systems and entire galaxies. These could all be analyzed using the same basic ideas that we talked about in the last episode. Then in 1933, Fritz Zwicki applied that logic to a much bigger system, the Comoma cluster. Now, galaxy clusters are huge.

[00:08:11]They're enormous collections of galaxies bound together by gravity and they are old systems. Gravity dominates their behavior. That makes them surprisingly clean labs. Zwicki measured how fast the galaxies inside the Koma cluster were moving. And then he asked a pretty simple question. Given those speeds, how much mass must the cluster contain to stay gravitationally bound? The answer was a problem. The galaxies were moving way too fast to be held together by the mass that's inferred from visible stars alone. By Zwicki's estimate, the cluster needed hundreds of times more mass than could be seen. Now, if that extra mass like wasn't actually there, then the cluster should have just torn itself apart long ago. To describe this discrepancy, Ziki used the term dunal mattery, dark matter. Look, it's important not to over interpret that phrase, okay? Because Wiki wasn't proposing things like exotic particles or new physics or anything. He just meant matter that was dark because it didn't emit much light. So things like faint stars or gas, dust, maybe even black holes, ordinary stuff that we just hadn't properly detected yet. Now, at the time, that was a really reasonable assumption. The result was largely ignored. Galaxy clusters were poorly understood. distance measurements were uncertain and Zuki himself, yeah, he had a bit of a reputation for making like bold claims and then just ignoring them himself. So, most astronomers assumed that better observations would eventually make the problem go away. For a long time, it was possible to dismiss this as just a quirk of extreme systems.

[00:09:59]But that option disappeared once the same kind of mismatch showed up inside galaxies. But that's for the next episode. Stay nerdy. Galaxies rotate and how fast they rotate tells us that they shouldn't be able to hold themselves together. Based on the matter that we can see, stars near the edges should escape. But they don't. This is Dark Matter Explained, episode 5. Galaxy Rotation. A spiral galaxy looks like at first glance like a fairly simple gravitational system. Most of the light and most of the matter that we can directly observe is concentrated near the center. That's where the stars are densest and the gas is brightest. As you move outwards, the amount of visible matter starts to drop off. Now, mass curves spacetime creating gravitational effects. And from this we know that stars close to the center of the galaxy should orbit quickly while stars further out where there's less curvature pulling them inward should orbit more slowly.

[00:11:05]This isn't a new idea or anything. It's the same basic reasoning that explains why planets further from the sun move more slowly in their orbits. Now, for a long time, astronomers couldn't properly test this in galaxies because the outer regions are really faint and measuring motion out there was really difficult.

[00:11:26]That changed in the 1960s and '7s with the work of Aver Rubin and instrument builder Kent Ford. You see, Ford developed a much more sensitive spectrograph. And this made it possible to measure how fast stars and gas were moving across entire galactic discs. So Reuben used it to map rotation speeds from the bright early regions all the way out to the edges. What they found was consistent across many different galaxies. Instead of slowing down, the orbital speeds stayed roughly constant with distance. stars far from the center were moving just as fast as stars much closer in. Now, if the visible matter that we can see was all of the mass that the galaxy contained, then those outer stars should not be gravitationally bound to it over long periods of time.

[00:12:20]They should eventually just drift away, but they don't. And because this behavior shows up again and again across different galaxies, it can't just be dismissed as some sort of quirk or like just a simple measurement error. It means that there is more gravitational influence present than the visible matter alone can provide. So in the 30s, Fritz Zwicki showed that the rotation versus mass of galaxy clusters wasn't adding up. And then by the 70s, Vera Rubin showed that the same thing was happening in individual galaxies. Now, if that was all, you might be thinking, you know, well, maybe the math is just wrong. But gravity doesn't just affect motion, it also bends light. But that's for the next episode. Stay nerdy.

[00:13:07]There's more than one way to work out the mass of a galaxy. We can look at how it rotates, or we can look at how it bends light. This is Dark Matter Explained, episode 6, gravitational lensing. Now, gravity doesn't just control how objects move through space.

[00:13:25]It also affects how light travels.

[00:13:27]According to general relativity, mass curves spacetime. And when spacetime is curved, light doesn't move in straight lines. It follows that curvature. That means that massive objects act a little bit like lenses. So as light from distant galaxies passes by them, its path is bent. Now the results can be distorted shapes. They can be stretched images uh like multiple copies of the same object. And this effect is what we call gravitational lensing. Now what makes lensing so useful is that it doesn't depend on how fast anything is moving. There are no orbits to model.

[00:14:06]There's no assumptions about any equilibrium. You just compare where the light appears to come from and where it should have come from. And from that difference, you can work out how much mass must be present along the line of sight. Now, when astronomers started doing this for galaxies and like galaxy clusters, they found something odd. The amount of mass needed to explain the lensing effect was a lot more than the mass that they could see. But more than that, the expected mass wasn't appearing where the light was. In a lot of the systems, the gravitational lensing maps showed large amounts of mass surrounding galaxies in these extended halos or like offset completely from the brightest regions. So measurements of rotation curves of galaxies and galaxy clusters had already shown that they were behaving as if extra mass was present.

[00:15:06]lensing gave astronomers a second and completely independent line of evidence.

[00:15:12]So now the same missing mass is showing up in measurements of motion and of light. But that's not all. There's another place it appears and it's in the early universe. But that's for the next episode. Stay nerdy. This is an image created by 13 billion-year-old light and it tells us that there is something missing in the universe or in our explanation of it. This is dark matter explained episode 7 cosmic microwave background. When we look out into space, we're also looking backwards in time.

[00:15:49]Light takes time to travel. So distant objects show the universe as it was in the past. Now, if you go back far enough, you reach a point long before galaxies or stars existed. And we have a direct image of that era. It's called the cosmic microwave background. It's a map of radiation released when the universe first became transparent about 380,000 years after whatever event created the universe. What's interesting about this image is that it's not smooth. You can see tiny variations throughout differences of about one part in a 100,000 and they represent small densities in matter, slightly denser regions and slightly less dense regions.

[00:16:38]Now those small differences are the starting point for everything that formed later. Now, if the universe contained only ordinary matter, the kind that interacts with light, those small density variations wouldn't grow very efficiently. In the early universe, ordinary matter was tightly coupled to radiation. Pressure from light resists collapse, slowing the growth of structure. When you model that situation, gravity simply doesn't have enough time to turn those tiny fluctuations into galaxies and clusters by the time we get to now where we can observe them. But when you include an extra component, matter that feels gravity but doesn't interact with light, the model changes. That extra matter can begin clumping earlier before atoms even form. When cosmologists run simulations with this extra component, two things line up, the detailed pattern of fluctuations in the cosmic microwave background and the large scale structure of galaxies we see today. So given what we know and understand now, stars on the outer edges of galaxies move too fast to be gravitationally bound to what we can see. And the same idea affects galaxy clusters. Gravitational lensing shows light bending around matter that isn't apparently there. And the map of the early universe is missing an extra component.

[00:18:12]The universe looks like it's missing some mass. So, what could this mass actually be? Well, that's for the next episode. Stay nerdy. The universe looks like it's missing some mass. And the simplest explanation is that we're just not seeing all of it. This is dark matter explained episode 8. Machos. The concept of dark matter shows up whenever we use gravity to measure how much mass is present when we're looking in galaxies, galaxy clusters, gravitational lensing, or maps of the early universe.

[00:18:46]Now, in all of those cases, gravity acts as if there's more mass than we can account for. So, the first and most obvious idea is, you know, it's just ordinary matter that doesn't emit very much light. We're talking about things like faint stars, cold gas, brown dwarfs, um, planets, even black holes, things that are hard to detect, especially at large distances. And this idea is grouped under the name machos, massive, compact halo objects. The universe is big. It's messy. It's filled with things that are difficult to observe. Missing some matter wouldn't be very surprising. But nuclear physics, cosmology, and observations of the early universe all agree on how much ordinary matter there should be. And it isn't enough. Even if we hid all of the matter there into macho objects, we would still fall so short of the amount of mass needed to explain what gravity is doing.

[00:19:50]Here's the way you can try to understand it. What we can see is about 5% of the mass energy content of the universe.

[00:19:57]Dark matter is about 27%. That's like around a five time ratio. Take our solar system as an example. Now, I don't know if you know this, but our sun contains 99.8% of the entire mass of the solar system. Like all of the planets, dwarf planets, um, asteroids, comets, everything else only makes up 0.2% 2% of the mass of the solar system. So, if there was matter in our solar system that was like too faint for us to be able to see, we're talking about five suns worth of matter. Now, I'm not saying that there are like five dark hidden suns in our solar system at all.

[00:20:32]Don't get me wrong, it's it's just an example. Look, if most of the missing mass was made up of compact objects like faint stars or black holes, we'd still expect to see their gravitational effects directly as they pass in front of distant stars. It would briefly magnify their light. Now, astronomers have looked for this and they just don't see it often enough. Absolutely, for sure, some of the universe's missing mass is hidden from view, but only a very, very small fraction. It just doesn't solve the dark matter question on its own. So, if the missing mass isn't just ordinary objects that aren't very bright, then what about a particle of some kind? Like maybe something very light that doesn't really interact with anything and that there's a lot of out there. But that's for the next episode.

[00:21:22]Stay nerdy. Trillions of nutrinos pass through your body every second and they barely interact with you at all. So, could they be responsible for what we call dark matter? This is dark matter explained, episode 9, nutrinos. We already know that the missing mass in the universe isn't just ordinary matter that we fail to see. So, what if it's a kind of particle and one that already exists? Now, nutrinos were first proposed back in 1930 by Wolf Gang Py to fix a problem in nuclear physics. But later Enrio Fermy developed the theory properly. Now at the time nutrinos were purely hypothetical. They weren't detected directly until the 1950s.

[00:22:11]What makes nutrinos interesting for the dark matter question is how they behave.

[00:22:16]They don't carry electric charge. They don't interact electromagnetically.

[00:22:23]They pass straight through most of the matter in the universe without noticing it. Nuclear reactions in stars produce enormous numbers of them and they zip out across the universe. Trillions pass through our bodies every second and almost none of them interact with a single atom. That all sounds really promising as a candidate for dark matter. A real particle that there's lots of and is practically invisible.

[00:22:52]But there are a few problems unfortunately. First of all is the mass.

[00:22:57]Now, nutrinos do have mass. For a long time, we thought that they didn't. But the mass that they have is incredibly small. And even when you add up all of the nutrinos that should exist in the universe, they simply just don't contribute enough mass to account for the gravitational effects that we see.

[00:23:14]Now, the second one is speed. You see, nutrinos move incredibly fast, close to the speed of light. And that matters because of how structure forms in the universe. fastm moving particles, well, they just don't clump easily. They stream out of dense regions instead of settling into them. And that kind of behavior would be what we call hot. When cosmologists model a universe dominated by hot dark matter, the results don't match what we see. Large structures would then form first and fragment later, which is like the opposite of how galaxies and clusters actually appear to have formed. So, while nutrinos do contribute a small fraction of the universe's missing mass, they can't be the main component of dark matter, they're just not heavy enough and they're not slow enough. But is there any other particle that could contend for the spot? Well, some new research has found something strange, but that's for the next episode. Stay nerdy. Is dark matter strange? There is new research that suggests dark matter doesn't have to be a new kind of particle. It could be something familiar after all. This is dark matter explained episode 10. Strange quark matter. Back in the 1980s, physicist Edward Whitten proposed a radical idea. Under extreme conditions, ordinary nuclear matter might not be the most stable form of matter. Instead, a state made of up, down, and strange quarks together called strange quark matter could be more stable than protons and neutrons. And if that's true, then small stable chunks of this material could exist. Now, these are usually called strangelletits or more generally quark nuggets. These clumps wouldn't form any atoms or emit any light. And because they'd be incredibly dense, even a small nugget could carry a lot of mass gravitationally, they'd kind of behave like dark matter.

[00:25:21]This idea appeared in the 80s. So why was it ignored? Well, cosmology tells us how much ordinary barionic matter exists in the universe. So if dark matter were made of quark nuggets formed in the early universe, it seemed like they would still count as ordinary matter.

[00:25:39]and there just isn't enough of it to explain what we see. But more recently, people have looked at this idea again.

[00:25:46]And new work has shown that if quark nuggets formed before barriians were fully frozen into protons and neutrons or if they formed during certain early universe phase transitions, they wouldn't be included in cosmologyy's measurements of ordinary matter. Now, if this was the case, then quark nuggets would be a really good candidate for dark matter. They would have mass, not interact with electromagnetism, and exist in the early universe. All the things we're looking for. But before you get like too excited here, quark nuggets don't answer all of the observations that we see. You see, they would still be like compact objects, um, not like tiny little particles. So if they were too massive or like too common, there was like too many of them, we would see their gravitational effects directly through microl lensing where background stars briefly brighten as a nugget passes in front of them or we'd see them through statistical distortions of distant sources. So taken together, these observations don't like rule out cork nuggets, but they put a fairly like narrow limit as to what size they can be, how much of them there can be, and the strength of their interactions.

[00:27:05]So if the matter that we know from black holes to quark nuggets won't exactly do the job, then maybe something entirely new will. But that's for the next episode. Stay nerdy. If dark matter is actually a particle, it has to do three things at once. It has to be massive. It has to barely interact with us. And it has to move slowly enough to clump.

[00:27:30]We might need a new type of particle for this. This is dark matter explained episode 11, wimps.

[00:27:38]Now, what we already know about dark matter um is that it can't be ordinary matter. it can't interact electromagnetically and it can't be fastm moving in the early universe. This ruled out things like machos, massive compact halo objects like brown dwarfs or black holes and it also ruled out fastm moving particles like nutrinos. So physicists started asking is there a kind of particle that naturally has all of the properties that we need? And this led us to the idea of WIMPs. Weekly interacting massive particles. The name is very literal. Massive means they would have enough mass to matter gravitationally. Weekly interacting means that they would barely interact with ordinary matter, making them effectively invisible. And particles with this kind of mass would have been slow moving in the early universe. So we could call it cold dark matter. What has made WIMPs especially attractive is that they're not invented just to solve the dark matter problem. In the early universe, when temperatures were extremely high, heavy particles could be produced naturally and then as the universe expanded and cooled, those particles would stop being created and their remaining number would then like freeze out. As it turns out, some extensions of the standard model, such as super symmetry, predict particles that interacted in the right way and naturally ended up with about the right abundance to explain dark matter today.

[00:29:14]Now, this was an exciting idea because it suggested that dark matter might not require exotic new theories, just particles that fit naturally into extensions of physics that were already being explored. So, for decades, WIMPs became the leading dark matter candidate. You see, if they exist, then they should occasionally collide with ordinary matter, producing tiny but detectable signals. That meant that the idea wasn't just theoretical, but it could actually be tested. So, physicists started building experiments to look for them. But if wimps are such a good idea, why have we never seen one? Well, that's for the next episode. Stay nerdy. If dark matter is made of particles, then the Earth and us are moving through them right now. So why don't we ever see anything hit us? This is Dark Matter Explained episode 12, searching for dark matter particles. Now, one reason particle dark matter became so popular is that it is testable. If dark matter consists of particles that have mass and barely interact with ordinary matter, then the Milky Way should be filled with them. And as the Earth moves through the galaxy, we should occasionally pass through those particles. And very rarely, one of them should collide with an atomic nucleus. Now the effect would be tiny, a small transfer of energy, but in principle it's measurable. The most likely candidates for a dark matter particle are called WIMPs, weekly interacting massive particles. So experiments were built to look for exactly that. They're placed deep underground because more shielding helps to block unwanted background particles.

[00:31:13]And one of the main goals is to reduce noise as much as possible.

[00:31:19]Inside these experiments, they are very clean and they have very stable materials such as liquid xenon or very carefully grown crystals. The detectors watch for small flashes of light or tiny recoils that could indicate a collision with a dark matter particle. The difficulty is that nature produces, like lots of other signals, radioactive decays in the surrounding environment, cosmic rays that slip through the shielding, and even nutrinos passing straight through the detector. So, the experiments don't just record events. They work really hard to rule out every known explanation first. Over time, the detectors have become larger and more sensitive, but so far they haven't seen a convincing signal. Now, that doesn't mean the experiments failed. It just means that the simplest versions of particle dark matter haven't shown up where we thought they might. Either the particles interact even more weakly than expected, or dark matter isn't made of that kind of a particle at all. So, if the most studied particle candidate keeps coming up empty, are there any other candidates out there? Well, that's for the next episode. Stay nerdy. Dark matter might not be heavy at all, it might be made of particles so light that calling them particles is already a bit of a stretch.

[00:32:52]This is dark matter explained, episode 13, axons. In particle physics, there is a puzzle about why the strong nuclear force behaves the way it does. Now, to fix that problem and and when I say fix, I mean mathematically, not actually, uh theorists proposed a new symmetry, and that symmetry just happens to come with a new particle called the axon. And it turns out that the axons would interact extremely weakly with ordinary matter. They wouldn't emit or absorb light. And even though they're incredibly light, they would still behave as cold dark matter in the early universe. Cold means slow.

[00:33:35]Now, we had to rule out nutrinos early on because they move too fast to match up with what we already know about how things form in the universe. Axons would be produced in the early universe in a way that leaves them moving very slowly overall, allowing them to clump and help form structure just like a heavier dark matter would. So axons tick the same boxes as wimps, invisible, weakly interacting, and cold, but for completely different reasons. They also come with a useful bonus. you'll have a bonus because axons would interact with electromagnetism in very specific ways.

[00:34:17]So they're testable. Now if axons exist, the galaxy should be full of them. And in the presence of a strong magnetic field, axons should occasionally convert into photons like radio frequency photons at very specific energy set by the axon's mass. So experiments are built to slowly scan through frequencies, effectively like tuning a radio signal to different possible axion masses and then listening for an extremely faint excess signal. Now other experiments can use different techniques as well, things like dialectric stacks, resonators, precision magnetic sensors, but they're all chasing the same thing, a narrow, persistent signal that shouldn't be there. So far, no confirmed detection. But as with the WIMPs, that's not the same as failure. It just takes time to search for a tiny signal in a really, really big space. Now, so far, we've treated dark matter as a single particle behaving more or less like on its own. Well, what if dark matter isn't just one particle? What if it's an entire hidden sector with its own forces and interactions?

[00:35:34]Well, that's for the next episode. Stay nerdy. Is there a hidden dark sector in our universe? Most searches for dark matter look at a single new particle, and that assumption drives how we design experiments and how we interpret data.

[00:35:49]And it's a very useful simplification, but there's no fundamental reason that it has to be true. This is Dark Matter Explained, episode 14, the dark sector.

[00:36:00]In the models that we've talked about so far, dark matter behaves like like a loner, right? It's a particle. Maybe it's a wimp. Maybe it's an axion. But does it have to be only one particle? In particle physics, the matter that we're familiar with, that doesn't come as a single particle. It comes as an entire sector. We have quarks, lepttons, we have forces, interactions, symmetries, all tied together. The dark sector idea is simply the suggestion that dark matter might be similar. Instead of there being one particle, there could be many. And instead of no interactions, there could be dark forces. Maybe dark matter could interact strongly with itself while still barely interacting with us. Now, that would change how we think about dark matter's role in the universe. For example, um dark matter particles interacting with each other could affect how dark matter clumps, how dense its halos are, or how it behaves in the centers of galaxies. Without contradicting anything that we see through gravity, this idea doesn't make dark matter any easier to detect. Dark sector models are built so that interactions with ordinary matter remain extremely weak. That's why dark matter could still have escaped detection in experiments designed for single weekly interacting particles. What it changes is the search strategy. The dark sector isn't one theory, right? It's a broad framework that's keeping our options open. So, at this point in our story, we know the universe is missing something.

[00:37:48]It might be a particle. It might be a sector of particles and interactions.

[00:37:52]These things are hard for us to detect and we haven't been able to find a way to see them yet. So, how do we even begin to test for them, but that's for the next episode. Stay nerdy. How do we actually look for dark matter? Like, we can't see it. We can't touch it. We think it's there and we have an idea of what kind of thing it could be, but how do you actually find it? This is dark matter explained episode 15 testing dark matter. Now most current searches fall into like threeish categories. The first is direct detection. If dark matter is made of particles then the galaxy should be full of them. And as the earth moves through the halo a tiny number should occasionally interact with ordinary matter. those interactions would be incredibly rare and incredibly weak, but not impossible to detect. Some of the most sensitive direct detection experiments that we have today are things like the Xenon NT and the Lux Zeppelin. The second approach is indirect detection. If dark matter particles can interact with each other, then they might occasionally annihilate or decay, producing ordinary particles such as photons, electrons, nutrinos.

[00:39:13]Astronomers use space-based detectors, things like the Fermy gamma ray space telescope, and then particle experiments such as AMSO2 on the International Space Station. The challenge here is that astrophysical environments are like really messy. So you've got pulsars, supernova remnants, cosmic ray interactions. They can all produce similar signals which makes interpretation hard even when something unusual shows up on a detector. Now the third approach is production in the lab.

[00:39:44]So if dark matter particles exist, they might be created in high energy collisions. So experiments like the Large Hydron Collider can look for things like missing energy, cases where momentum and energy don't balance, suggesting that something invisible carried some away. What all of these approaches have in common is that they don't test a single theory. They test broad classes of ideas. Now, so far nothing conclusive has been seen. But that doesn't mean that nothing has happened because each null result doesn't mean dark matter is wrong. It just means that the space of viable possibilities get smaller. Large regions of possible particle masses and interaction strengths have already been ruled out. And that's really useful information because it tells us what dark matter can't be. It might not be the answer that you want, but it is progress. And this leads us to a question that some people have. Do we just keep refining the same searches, or do we need to rethink the problem more radically, but that's for the next episode. Stay nerdy. Hey, what if gravity is wrong?

[00:41:00]Stars in galaxies orbit too fast for the amount of matter we can see. We can explain that by adding dark matter or we can change our understanding of how gravity behaves. This is dark matter explained episode 16. Mond. Mand stands for modified Newtonian dynamics. Now it was first proposed in the 80s to explain the relationship between the visible matter in a galaxy and how fast stars orbit within it. In Newtonian gravity, the gravitational acceleration a star feels depends on the amount of mass inside its orbit. If most of the mass is concentrated near the center of a galaxy, stars further out should experience weaker gravitational acceleration and they should orbit more slowly. However, Vera Rubin showed that in real galaxies, orbital speeds stay roughly constant far beyond where most of the visible mass is located. So instead of adding dark matter to explain this, Mand modifies the relationship between force and acceleration. Above a certain acceleration scale, gravity behaves exactly as Newtonian physics predicts. Below that scale, the relationship changes. The effective gravitational acceleration becomes stronger than Newtonian gravity would predict for the same amount of visible mass. That acceleration scale is extremely small. It's like around 10 the -10 m/s squared. And it turns out to be the typical gravitational acceleration experienced by stars in the outer parts of galaxies. Now once you apply this modification, the observed galaxy rotation curves follow directly from the distribution of visible matter. No dark particles required here. Now that sounds exciting and promising. Of course it does. Doesn't that explain the case of missing mass without needing dark matter? It's a great thing. Well, no.

[00:43:07]You see, the dark matter problem isn't confined to just galaxy rotations. They are one measurement that implies missing mass, but they're not the only measurement. And when you apply Mand beyond individual galaxies, it just doesn't work. Galaxy clusters still show missing mass and it doesn't explain gravitational lensing and it can't give a consistent description of the early universe. But man is not a complete theory. In its original form, it modifies Newtonian gravity, but it doesn't provide a relativistic description. If gravity really is modified at low accelerations, then is there a way to make it consistent with relativity?

[00:43:53]Well, that's for the next episode. Stay nerdy. It's possible that we're wrong about how gravity behaves at the edges of galaxies.

[00:44:02]If we are, then we might not need to search for dark matter at all. Problem is, we can't only change gravity for how stars move. We also need to explain how light bends, how clocks tick, and how the universe evolves. This is dark matter explained, episode 17, relativistic mand. Now, the original version of Mand, which is modified Newtonian dynamics, modifies Newtonian gravity. That's enough to describe the mismatch we see in galaxy rotation curves without needing dark matter. But Newtonian gravity is not our fundamental theory. Gravity is really described by general relativity. General relativity doesn't treat gravity as a force. It describes it as the curvature of spaceime. And that curvature controls the motion of matter, the bending of light and the expansion of the universe.

[00:45:01]So if Mand is something real, then it has to be embedded in a fully relativistic theory. Now the most well-known attempt to do this is a theory that's called teas. It's short for tensor vector scalar gravity. In teas, I'm just going to call it that. uh spacetime is still described by a tensor field like in general relativity but two additional fields are introduced. We have a scalar field which modifies how gravity behaves at low accelerations and then we have a vector field which helps control how time and space are measured together. These fields are arranged so that in high acceleration environments, gravity reduces to ordinary general relativity and then in low acceleration environments, Mandlike behavior emerges.

[00:45:53]But in galaxy clusters, even tea still requires additional unseen mass to explain the observed gravitational effects. In cosmology, matching the detailed structure of the cosmic microwave background requires adding extra components such as like massive nutrinos or other fields. At that point, like the theory is no longer explaining the data using only modified gravity.

[00:46:22]It's doing it by introducing new parameters that contribute to gravity in pretty much the same way dark matter would. This isn't to say that relativistic mind couldn't be the answer to the dark matter problem, but so far it's showing us how hard it is to replace dark matter entirely without reintroducing something that behaves a lot like it. So, which do we think is the most likely candidate, dark matter or mind? We'll talk about that in the next episode. Stay nerdy. When something goes wrong in physics, you usually have like two options. You can add something new or you can change the rules. Dark matter and modified gravity are those two options. This is dark matter explained episode 18. Dark matter versus modified gravity. Modified gravity changes how gravity behaves at low accelerations. Dark matter keeps gravity unchanged but adds new matter that we can't see. Inside individual galaxies, both approaches can work pretty well.

[00:47:27]The difference shows up when you look at systems where matter, light, and gravity can be separated. Now, one of the main examples is something called the bullet cluster. The bullet cluster is a collision between two galaxy clusters and most of the ordinary matter in each cluster is hot gas. This emits X-rays.

[00:47:49]During the collision, uh that gas slows down and piles up in the middle. The galaxies themselves pass through each other with very little interaction. But when astronomers map the gravitational field of the system using gravitational lensing, the strongest gravitational pull isn't found where the gases. It's where the galaxies are. In a dark matter picture, this makes total sense. Dark matter passes straight through the collision just like the galaxies and continues to dominate the gravitational field.

[00:48:25]In modified gravity theories though, this is much harder to explain because if gravity is determined primarily by the visible matter, then the strongest gravitational signal should track the gas, but it doesn't. You can try to fix this by adding extra fields or extra components, but once you do that, you're effectively adding something that behaves like dark matter. The same pattern appears in cosmology. Dark matter fits naturally into models of the early universe. It explains the structure of the cosmic microwave background, the growth of galaxies over time, and the large scale distribution of matter using a single additional component. Modified gravity though that can be made to work in some of these contexts but only by adding extra ingredients that play a role very similar again to dark matter. So it seems that the dark matter idea explains more of the observations that we've seen in a simpler way but we haven't seen any evidence for it. While modified gravity explains some of the observations very well but struggles to scale up without becoming very complicated. The best that we can do for now is to focus on what we can actually test or to come up with an entirely different idea altogether. But that's for the next episode. Stay nerdy. We're going to talk about a new theory of gravity. This is Dark Matter Explained episode 19.

[00:50:00]Emergent gravity. Most of the discussion around dark matter is framed as like a choice between two options. Either there's unseen matter in the universe or gravity behaves differently than we think. But some researchers think that framing might be too limited. Instead of asking what dark matter is or how gravity should be modified, they're saying what if gravity itself isn't fundamental. In these approaches, gravity isn't treated as like a basic force like we do when we say it's one of the four fundamental forces like electromagnetism.

[00:50:36]It's treated instead as a collective effect, something that emerges from a more basic ingredient like in the same way that temperature emerges from the motion of atoms. Now, there isn't just one version of this idea. There's actually a family of approaches and they have different ideas within them about what gravity is supposed to emerge from.

[00:50:59]Some of them are informationbased and in these models gravitational effects would arise from how information or entropy is organized in spaceime. The extra gravitational pull we usually attribute to dark matter would then be a consequence of how spacetime responds to ordinary matter on large scales. Now other approaches are inspired by holography in quantum gravity. In holographic theories, a gravitational system inside a region of space can be fully described by a different theory defined on its boundary. one that doesn't contain gravity at all. Now, there are also condensed matter style ideas where spacetime itself behaves like a medium. Gravity in this view is similar to like sound waves in a material, a real measurable effect that isn't a fundamental interaction.

[00:51:56]different approaches, but they're all trying to explain some of what we see in galaxies without introducing new dark matter particles. But none of them are complete ideas. Now, currently, they don't reproduce gravitational lensing reliably. They struggle to match the detail structure of the early universe and most of them don't have a complete underlying theory that's needed to be able to produce clear and testable predictions across all scales. At the moment, these ideas are all exploratory.

[00:52:31]They're speculative. They're not replacements for dark matter or for modified gravity. their attempts to understand whether we're looking at the problem from the right angle. Which brings me to our final question. What would it actually mean to solve the dark matter problem? But that's for the next episode. Stay nerdy. Dark matter is one of the biggest problems in physics because it keeps showing up in completely different measurements. This is dark matter explained. Final episode.

[00:53:03]Where things stand. Across this series, we've seen the same mismatch appear again and again. When astronomers study galaxies, galaxy clusters, gravitational lensing, and even the early universe, gravity behaves as if there's more mass present than we can account for. Well, not with the matter that we can see anyway. Maybe it's just ordinary stuff that's hard to see. Faint stars, gas, black holes. We call them machos. But we can estimate how much ordinary matter exists using nuclear physics and early universe cosmology, and it just isn't enough. Maybe it's a particle we already know, something like a nutrino, but they're too light and they move too fast. And so we started to search for new things, things like wimps, and then axons, because they fit the conditions, and we can actually build experiments to test for them. But so far, direct searches haven't given us a detection.

[00:54:00]Now, that doesn't kill the idea, but it tells us that the simplest versions aren't showing us what we're hoping to see. So, maybe we need to pivot to a dark sector, a whole region of dark matter with its own forces and interactions that we barely couple to.

[00:54:16]Or maybe what we actually need is to modify gravity. Mond and relativistic versions like Tez might work for galaxies, but they don't really work for clusters. They don't work for lensing and they don't work for cosmology. It may even be that we need a combination of both dark matter particles and modified [clears throat] gravity to explain what we're seeing. But why stop there? There are even newer exploratory ideas like emergent gravity which try to change what gravity is in the first place. Really interesting ideas but like nowhere near complete. So that's where we are. From the first observations that something was off by Zwicki back in 1930 to deep underground facilities, advanced satellite telescopes, and entirely new ways of thinking about gravity. We may not have found the answer yet, but don't lose hope. Research is a process. It takes time. Even null results gets us closer, and every new lab built brings us new technology. Now, most of physics focuses on small incremental changes.

[00:55:24]These bring us further into the future, but some of it focuses on like big weird unanswered questions that make all of our brains get a little bit twisty. I hope that this series has helped dispel some of the confusion and give you like a good solid idea of where the dark matter idea came from, what it might be, and what we're doing to find the answer.

[00:55:48]Thanks for following along. I'll be back soon with a new series.