Is dark matter real? - Why can't we find it? - physicist explains | Don Lincoln and Lex Fridman

Ajouté : 2026-06-01

[00:00:03]But there is the what you call the worst prediction in physics which is >> oh yeah that's another one >> a nice little insight about the complicated nature of dark energy. So the observations as you described say that empty space has a tiny energy density that accelerates expansion of the universe.

[00:00:24]But quantum field theory's prediction for what vacuum energy should be when coupled with gravity is much larger.

[00:00:34]>> Mhm.

[00:00:34]>> Uh so this is what makes for the uh quote you have a video on this worst prediction in physics.

[00:00:41]>> Can you can you explain this crisis?

[00:00:43]>> Well the there's a measurement and you can measure how fast the universe is expanding and from that you get a measurement of dark energy. However, if you then say, well, suppose the dark energy is due to fields in space. So that's quantum field theory. Hey, I know a lot about quantum field theory.

[00:01:03]>> And so we can take the quantum field theory and we can calculate what the density of energy is due to quantum field theory. And basically what you do is you take within a volume the uh all of the wavelengths, the the longer wavelength, the shorter wavelengths, the shorter shorter and shorter. And you can add them all up.

[00:01:26]And each wavelength adds a certain amount of energy. And if you add that all up, then you get a number. And that number is the rather embarrassing 10 to the 120 power times that's a one with 120 zeros after it bigger than the measurement of dark energy.

[00:01:50]>> Yeah.

[00:01:51]>> So you go yuck that is not fun at all.

[00:01:56]And that is because the equation comes to the highest energy or the smallest wavelength particle that you can imagine to the fourth power since anything to the fourth power is a big deal. So that's where you get that awful number.

[00:02:13]Now if it turns out that there is some new physics that's just about at the energy scale we can measure using our biggest particle accelerators. Remember I told you that that was a factor the maximum energy scale plank scale is 10 the 15 times bigger than what we can measure now. So let's say that we don't have to calculate up to the plank scale because something happens something changes at the energy that we know right now. Well then that means we don't have to integrate to plank scale we integrate to 10 the 15th less of the plank scale and this thing is to the 4th power. So 10^ the 15 to the 4th power is 60. So now even if we say you know Don he's brilliant he's going to find something at the LHC tomorrow is going to solve all this problem. Now we've solved it.

[00:03:04]It's much better. It's only different by 10 to the 60 power which is still pretty bleeding big. So the short answer is there is very clearly something going on something wrong very badly wrong in the quantum field theory. you know, we have to have maybe there's another field that balances out the energy that cancels it down. And even that, you know, that that's not so so outrageous. You know, you could imagine that there's another, you know, like we have matter and antimatter. They balance pretty well.

[00:03:38]Okay, maybe there's something going on.

[00:03:39]You could cancel that out. That'd be perfect.

[00:03:43]Cancelling something to zero is easy cuz, you know, plus one and minus one 0 + 2 - 2 0. But we still have dark energy. Dark energy is a little bit. So if it cancels, it doesn't cancel exactly because it left over that little bit of dark energy. So that is its own curiosity. Perfect cancellation pretty easy. Theorists do that, you know, eight times before breakfast. Imperfect cancellation much harder.

[00:04:10]>> Just to elaborate that a little bit, what do you think solving, in quotes, solving dark energy would look like?

[00:04:16]Well, you could what you would do is you would hypothesize that there existed some other field that had the the the reverse uh effect of existing quantum fields >> but not to zero >> but not to zero. So, but if you had it to go to zero, you know, uh sure, maybe there's a field that that exists at really high energies that we haven't seen yet. I don't know, but it cancels things out and we're cool.

[00:04:40]>> How would we then demonstrate the existence of that field? Uh well that would depend on the the prediction.

[00:04:46]>> How do you even come up with a new field >> like all theorists do? Well, let's add something to my equation and see what happens. I mean and and that's okay. I mean I I'm being glib about that but that is precisely what you do. You say what change we we have this thing that works quite beautifully except it fails here. What is the addition that we need to make that changes very little in the realm that we measured and yet fixes this hard thing?

[00:05:14]>> And so you literally just go d okay what do I need plus six or something and as long as it makes no changes where it would hurt our measurements and fixes the big thing then that is at least a candidate theory. Now, that doesn't mean it's right, but it at least gives you an understanding of what the right answer should look like.

[00:05:41]>> And so that's the first step is what should the real answer look like or what is a possible real answer. And then once you kind of know that then other people can look and say well let me think about a theory that kind of has the required properties to do what we need it to do.

[00:05:59]So it's a multi-step process but the first step is how do we tame this problem without coming up with really terrible predictions that we've already ruled out.

[00:06:14]>> And and so that's what you do. And and you know that that is literally a a sensible viable theoretical thing you know cuz you have to explore cool ideas.

[00:06:24]>> I mean one of the reasons dark energy is super interesting is it kind of gives us a mechanism by which we can talk about the deep future of the universe. I mean it's making we have observations about the expansion of the universe but it's also giving us the mechanism of that right. So we can talk about >> uh any weirdness any good model we have that that captures some of the weirdness of dark energy might give us insights about how this thing ends how the universe >> about the deep future of the universe.

[00:06:58]Right.

[00:06:58]>> Absolutely. as it stands right now. If dark energy is real and who knows, you know, if it's real exactly as we've measured it, then as the um universe gets uh bigger and bigger, dark energy becomes a bigger and bigger component of the energy balance of the universe and it takes over and it drives the continued accelerated expansion of the universe.

[00:07:23]And if dark energy gets lower, you know, for some reason that we don't understand, maybe it changes over time, gets smaller, that could change things.

[00:07:30]If it gets bigger, it could change things.

[00:07:32]>> That is one of the big open questions, whether it's constant over time or not, >> right? And there has been a recent measurement that suggests that dark energy is getting smaller. Um, however, that is a new measurement, not confirmed, blah blah blah blah blah.

[00:07:46]Nobody should believe it, but it's a hint that maybe it's changing, which is kind of cool in itself because the current bias until recently is that dark energy is constant. Now, I want to be super careful because it's misleading.

[00:08:02]People say dark energy is constant. Dark energy is a density.

[00:08:08]>> Now, that think about that. You have a certain density. Let's start with that.

[00:08:12]>> Then the universe expands. So energy is volume times density. If the universe gets bigger and the density is constant, that means dark energy is increasing.

[00:08:24]It's not just increasing as a fraction and overwhelming ordinary matter. But ordinary matter as the universe expands, its density decreases because it's constant and the volume gets bigger. The density drops. Dark energy until recently is thought to be constant density. So that's what's implied when you say constant. You say constant density which means it's actually increasing because space is increasing.

[00:08:50]The size of space is increasing.

[00:08:52]Interesting. And >> so that's a a weirdness. And that then ties into the nature of space. Why does that tie into the nature of space? Well, because if dark energy is a field in space, if you increase the volume, you would think the energy density would drop. Mhm.

[00:09:09]>> But if space is increasing and space is quantized, and I don't know if it is, then maybe what's happening is space isn't stretching, but like little space particles are appearing as the space, you know, there's like bubbles of space appearing. And each bubble contains a certain amount of dark energy. And so therefore, that would give you a sense that dark energy is a property of space rather than a field in space. But that's all very handwavy, guessworky stuff.

[00:09:41]>> So if you had bet all your money, is dark energy like a real physical, what does that even mean, thing that exists versus is this just an a renaming of the cosmological constant?

[00:09:55]>> Unfortunately, I think it's both. I mean, >> well, I mean, it is it it's describing a reality, but it's also maybe telling us something about space, >> literally a property of space.

[00:10:08]>> Yeah, it's I mean, that's kind of what it looks like given that it seems to be constant density. That seems to me, now this is not something anybody should believe.

[00:10:19]Please, nobody believe this. But it seems to me that this is leaning towards the idea that A it's a property of space, B, space is quantized, C as space is expanding little quantum of space are appearing and D each one of those quanta has a certain amount of energy associated with it and that would kind of explain the constant density. Now, please that's not anybody should nobody accepts that. This is just nonsense.

[00:10:52]>> But a lot of the stuff that you just said is experimentally probably experimentally testable. You can probably construct >> experiment the bubbles of >> well finding out the bubbles of space.

[00:11:04]But those quanta conceivably are planksiz bubbles.

[00:11:08]>> Yeah, the quanta.

[00:11:09]>> Well, they'd be quantum of space. I mean the idea is you know you look at a sand dune and it looks smooth and continuous but you can see individual grains of sand right and so what this is saying is as this dune expands new grains of sand are appearing and each one of them is a quantum of space.

[00:11:26]>> So what kind of experiments can we do in the coming decades or centuries to understand dark energy better? I mean people have been talking about quantum entanglement of gravity.

[00:11:39]In standard quantum mechanics a particle can be in two places at the same time.

[00:11:43]All right? So now you have two particles. So this particle can be in two places in the same time. This particle be be two places at the same time. You put them near one another.

[00:11:52]Well, if they're close to each other, there's a certain gravitational force.

[00:11:55]If they're far apart, they're certain.

[00:11:57]And if they one is close and one is far, you have another one. You can calculate the effects of gravity having to do with quantum entangle particles being in two places. And people are talking about doing this and trying to see if in doing such a measurement they might be able to definitively determine whether gravity is a quantum phenomena or a continuous phenomena. And that is potentially a measurement that could be done soonish because the technologies of inherent in all of this recent work on quantum mechanics is allowing people to be able to make instrumentation that might be precise enough to do this measurement. Now this will not tell us what quantum gravity is. It will not tell us anything. But it will tell us that gravity is quantized.

[00:12:59]And just knowing that, well, for one thing, it shuts out a whole realm of of continuous gravity. And the theoretical community will then turn its attention, forget this stuff, and and think over here. Now, that doesn't tell you that space is quantized, but it tells you that gravity is quantized if it bears out. So, and if gravity is quantized, then people will start thinking more about space being quantized.

[00:13:25]>> I have to ask because you mentioned dark matter is perhaps even more mysterious than dark energy.

[00:13:30]>> Okay.

[00:13:31]>> Can you can you can you uh build up the intuition why it's more mysterious. What is dark matter?

[00:13:36]>> Oh gosh, what is dark matter? A, I don't know. B, it's terribly fascinating.

[00:13:41]>> Yeah.

[00:13:42]>> All right. So first thing and the most important thing cuz I'm an experimentalist by God. The first thing is why do we believe there's dark matter? And the reason is that astronomical measurements do not agree with predictions by Newtonian or relativity theory. Galaxies spin too fast. Clusters of galaxies move too quickly. and the distortion of very distant galaxies due to the gravitational field of nearer galaxies disagrees with the prediction from what we see from the observed matter. So there are three very distinct reasons why we are predicting that that something is wrong in our understanding of either the laws of physics or the matter budget of the universe. The easiest one to talk about is the spinning galaxies. Now, this is what I'm saying is not unique to spinning galaxies, just easiest to talk about.

[00:14:40]So, galaxies are observed to spin more quickly than they should if we add up the gravity we see. By all rights, galaxies spinning that fast should blow themselves apart, and they don't. So, what can be the answer? Well, you have the force required for a star to orbit to move in a circle and you have the force due to gravity and they're connected by an equal sign and the prediction is wrong. So either the force due to gravity is wrong, the force needed to move in a circle is wrong or the equal sign is wrong. I mean this is really simple. One of those things is wrong.

[00:15:20]So one possibility is simply that Newton's law of gravity mass time the mass over r^ 2 * a constant that's just wrong. Another possibility is Newton's F= ma that we are taught in introductory physics is wrong. Both of those are eminently possible over here. Maybe we don't understand gravity or maybe there's more mass than we can see. So these, you know, I mean, it's nice that that you can look at this really simply and come up with a list, you know, cookbook things we can test.

[00:15:54]And um and so we've done that. We've gone and said, what are the possibilities? Well, the most obvious possibility is that there is more mass than we can see. There's black holes.

[00:16:06]There's uh um hydrogen gas that we can't see. Whatever. There's something out there. So that was the first thing. So, you go and you look and there's no hydrogen gas because we can see that through radio waves. That's not it. Um, in the '9s we went looking for black holes, rogue planets, things like that.

[00:16:24]Those exist, but not enough of them.

[00:16:26]That's not it. And so now we're left with there's some sort of matter that we can't see or we don't understand gravity or we don't understand inertia.

[00:16:37]Now I personally if you asked me this oh I don't know 25 years ago I would have said the most likely uh answer is that we don't understand inertia or gravity you know I if 20 years ago 25 years ago that's what I would have said no problem however there have been a couple of observations that um that have caused me to change my thinking and I think that dark matter is more likely One of them is called the bullet cluster. So the bullet cluster there are two large clusters of galaxies.

[00:17:14]In these large clusters of galaxies, well any galaxy consists of a couple of components. There are the galaxies themselves. There is the hydrogen gas that surrounds the galaxies and maybe there is dark matter.

[00:17:28]And if dark matter is real or dark matter is not real, you will get different answers. If those two galaxies pass through one another, the galaxies themselves should pass through one another basically not interacting. But the big thing is the gas clouds. So if there's big clouds of gas, as the galaxies pass through one another, the clouds should interact and the gas cloud should stop in the middle and be really really hot. So then you would see if there were no dark matter, you would see a cluster of galaxies, cluster of galaxies, a big gas cloud in the middle.

[00:18:00]And because the big gas cloud in the middle is much more massive than the galaxies themselves, you would expect to see distortions that we call dark matter distortions in the middle. If however dark matter is real, the galaxies pass through one another. The cloud stops.

[00:18:18]Dark matter doesn't interact with the clouds. So it passes through. In that case you would expect to see the distortions where the galaxies are >> and that's what we see. So that is a strong evidence in my mind. The bullet cluster is strong evidence that dark matter is a real thing. And there is another example which is much more recent. Dark bullet cluster was a while ago called the dragonfly galaxies.

[00:18:40]There's dragonfly 2 and dragonfly 4.

[00:18:43]These are galaxies that rotate exactly according to Newton's laws.

[00:18:50]And so the fact that they rotate exactly according to Newton's laws says that whatever is causing galaxies to rotate too fast is not a property of matter. But if you had a galaxy where there was no dark matter, for whatever reason, it got stripped off or something, this is one of those lovely ironies that the existence of a galaxy with no dark matter is very strong evidence that dark matter is real because you can take the dark matter out. So the DF2 and DF4 also suggests to me that dark matter is real. So now while it remains possible that um we need to modify the laws of inertia or we need to modify the laws of gravity those are possible still in my opinion and now this is Dawn's opinion but it's probably the opinion of most of the the scientific community dark matter is likely a real thing. Now that's great. I've taken you all the way to dark matter. So now you're going to ask me. You're going to say, "Don, what is dark matter?" I'm going to I don't know, but I know what it isn't. Okay? I know that it is not black holes. I know that it is not rogue planets. I know that we've done the measurements. We've looked across nearly every mass range for compact objects and ruled them out.

[00:20:09]So if dark matter is real, it can't be made of those.

[00:20:14]So then you're left with the idea that dark matter is a particle. And that's what we've thought about. The name for the the dark matter particle that we've called for a long time is a wimp for a weekly interacting massive particle. And we have spent the last god 30 years looking for them in a various ways.

[00:20:30]There are three ways that we might see dark matter. The direct way which says that dark matter exists literally everywhere in this room in our laboratory and the dark matter is passing through the earth like a wind.

[00:20:43]And we put up detectors trying to see those. We have done that and we've seen nothing.

[00:20:48]>> So we should say we have done that for nutrinos.

[00:20:51]>> We've done that for many different types of dark matter. We just simply put detectors in labs deep underground and we can see nutrinos in them. It's true.

[00:21:01]But dark matter would have especially heavy dark matter. What these these WIMPs um they have a different signature and we've seen no evidence of dark matter interaction in these detectors.

[00:21:12]So nutrinos are also weakly interacting and also have mass >> they are >> but not enough ma. So wimps are >> heavy on the um >> right? Nutrinos are indeed wimps of a sort. Now we have to be careful what we mean by wimps. They are weakly interacting massive particles. But we can calculate and there's just not enough mass in them. It's not it.

[00:21:33]>> Got it.

[00:21:33]>> So we need another form. And we have seen zero evidence of this wind of dark matter through the uh the earth. Another possibility is you look where you think dark matter might be concentrated at the center of galaxies. And if dark matter exists and there's antimatter dark matter, maybe they annihilate and make photons. And so we look for gamma rays and various other signatures of annihilating dark matter. And there are always constantly announcements of oh we saw it oh we didn't oh we you know the problem is that way of looking for dark matter is hard because there are other ways of making for instance gamma rays like neutron stars and stuff and you really need to understand the details of galaxies really really well to believe that and then the final option is what I do where we smash particles together at high ma or high energy we try to make dark matter particles If you make dark matter particles because they don't interact except via gravity, they escape with your detector.

[00:22:36]So what you're seeing, what you hope to see is an event where you collide particles, a dark matter particle escapes and you don't see it, but the recoil you see on the other side because momentum is conserved. So you see a blob of energy on this side, nothing on the other side. Maybe that's dark matter.

[00:22:53]And that also happens with nutrinos. So you need to understand everything about nutrinos and calculate how many of those you see. and then hope you see more and then that might be dark matter. Again, that hasn't worked. So, we've ruled out some dark matter particles. But the problem is the range of space of possible mass. If dark matter is of a particulate form, the range of viable dark matter ranges from something like the mass of an asteroid to far lighter than an electron and everywhere in between.

[00:23:27]And we have looked, we've ruled out some little spots in that phase space, but that's a big range.

[00:23:33]>> Is it really possible to miss a particle the size of an asteroid?

[00:23:37]>> The astronomical searches were not sensitive to that level of of dark matter. But, you know, then you would expect that there would be some of those in the solar system. And if they're what we think like asteroids or something, then we'd heat them up and we'd eventually see them. But if they're really like truly dark matter doesn't interact with matter, which means they wouldn't absorb energy from the sun. So they'd be really dark. I don't know, maybe they're out there. But the only way we have we searched for them was um a thing called microlensing. So if a massive object, you have a distant star and a massive object pass between that star and your eye, that star will momentarily brighten.

[00:24:24]>> Mhm. And so you just look for these what they call microlensing events and you count them and you see some and we did see some. You know, black holes pass in front of stars and and we've seen them, but we just haven't seen enough. And for very low mass particles like asteroids, they um they just wouldn't make enough uh brightening effect to see. So there's like a minimum sensitivity of brightening and that about a third the mass of a moon. Our moon is about the sensitivity that we had. So you know that nobody I think really thought that these low mass guys were likely. what they thought was more likely. They were just unseen black holes, which I thought, you know, I think is completely reasonable. Then when that got ruled out, I thought, okay, modified gravity or uh or inertia. Well, now that you know, bullet cluster and dragonfly seems to have ruled that out. So, I'm stuck in my head with dark matter seems to be real, and it we don't know what it is.

[00:25:26]>> And it makes up a giant percentage of matter in the universe.

[00:25:29]>> It is five times more prevalent than ordinary matter. Wow, this is incredible. This is so fascinating and that's why it's cool. So, if someone out there is, you know, a young person wants to get into this, understanding dark matter is a big deal. I mean, it's five times more prevalent. The problem is is, as I told you, if the mass is ranging from an asteroid to far lighter than an electron, if you get on an experiment that looks at one little range of mass, maybe you weren't the lucky guy that measured the right place, you know?

[00:26:01]And that's one of the reasons why, as fascinating as I think it is, I'm not doing dark matter experiments because, you know, if you make an experiment that searches one mass range, it'll be blind to another mass range. So, what you need is you need many groups doing all sorts of radically different experiments, exploring all sorts of parameter space.

[00:26:23]And with all that said, until you see it, there still is the possibility that maybe we don't understand gravity or inertia, right? You know, you can't rule that out.

[00:26:34]>> If there is dark matter out there, you're hoping is actually somehow detectable.

[00:26:39]>> I mean, I don't know what it is. I think it's cool. It's very, very fascinating.

[00:26:43]That is one thing I really do hope in my lifetime is is understood because I' I'd like to know the answer to that. And that that's the thing that you could there legitimately you can see a discovery of >> you got to get lucky though I mean you got to look in the right place whatever it is >> just imagine or you have to come up with that really cool theoretical idea that everybody's overlooked which is another possibility and there are people who are really really religiously hating dark matter largely because we've looked so hard for so many years and the experiments in today's world are a million times more sensitive than when I was a a starting student and they still haven't seen anything. And that's why people really hate dark matter. I mean, some of them because they think we should have seen it by now, but >> you know, uh I don't know.

[00:27:32]>> I mean, I'm a sucker for direct observation. Not indirect is obviously also really great, but direct. Just imagine pointing your telescope in a certain direction and because of some artifact of cosmology being able to directly detect a giant amount of of a thing that you could say is dark matter.

[00:27:50]>> Yeah. You would see it orbit things orbit it or it would eclipse things in front of it or >> Yeah. like in an obvious way cuz uh some of the stuff you mentioned with DF2 and DF4 those are like brilliant indirect um deductions that there should be something like dark matter but some obvious yeah blocking oluding this kind of thing we did that in the 90s experiments called macho ogle and some others they looked for a black hole that you just can't see you know a black hole you can't see it's perfect it's a perfect candidate for dark matter. And if there's enough of them out there, remember there's five times the number of stars, which means there's a whole lot of freaking black holes out there.

[00:28:35]We should have seen them and we didn't.