Scientists Finally Solved Why Andromeda Is Heading Our Way

Added: 2026-06-01

[00:00:00]68 miles every second. That's how fast the Andromeda galaxy barrels toward us right now. The universe keeps expanding and galaxies usually run from each other, but Andromeda charges straight at the Milky Way like it picked a fight.

[00:00:15]That motion never made sense until now.

[00:00:20]Most galaxies behave like dots painted on a balloon.

[00:00:24]Blow the balloon up and every dot moves away from the others. That's how cosmic expansion works. [music] Space itself stretches, carrying galaxies with it, but Andromeda ignores that rule. It approaches us at roughly 150,000 miles per hour and that puts it on [music] a direct collision course with our galaxy.

[00:00:44]Scientists knew about the future crash for years, but they never fully understood why Andromeda overpowered the universe's expansion in the first place.

[00:00:53]Now, they figured [music] out that both galaxies sit inside a massive flat sheet of dark matter. You can't see it, but you can see what it [music] pulls on as it has gravity.

[00:01:04]Instead of forming a neat ball around the galaxies, [music] this dark matter spreads out more like a gigantic pancake, stretching across millions of light-years.

[00:01:13]And that flat shape changes everything.

[00:01:16]Gravity works like tension [music] in a stretched rubber band. Put a bowling ball on it and the fabric dips. Roll a marble nearby and it curves toward [music] the heavy ball.

[00:01:26]The Milky Way and Andromeda sit on the same stretched sheet of dark matter.

[00:01:30][music] That sheet doesn't pull evenly from all directions like a sphere would.

[00:01:34]It pulls more strongly along its flat [music] plane.

[00:01:38]That uneven tug slows the outward motion of nearby galaxies and pulls Andromeda inward.

[00:01:44]And space doesn't just contain galaxies, but also enormous empty [music] regions called cosmic voids. These regions expand faster than average because they hold less matter. When they stretch outward, they push matter toward their edges and build dense [music] walls of galaxies and dark matter around them.

[00:02:04]The Milky [music] Way and Andromeda sit near one of these cosmic walls. The faster the void expands, the more it [music] funnels matter into the surrounding structure, and that reshapes how gravity flows through our neighborhood.

[00:02:18]>> [music] >> So, instead of two lonely galaxies drifting toward each other by accident, it's a whole landscape that's shaping their motion.

[00:02:25]Dark matter forms the [music] terrain.

[00:02:27]Voids carve out valleys and ridges.

[00:02:30]Gravity acts like water running downhill. [music] Andromeda doesn't just decide to approach us. The entire structure of our local group, that's the small family of galaxies that includes us, guides its [music] path.

[00:02:43]That local group stretches about 10 million light-years across.

[00:02:47]Each light-year equals nearly 6 trillion miles. Even at 68 miles per second, Andromeda still needs about [music] 4.5 billion years to reach us.

[00:02:58]That sounds comfortably distant, [music] but in cosmic terms, that's basically tomorrow.

[00:03:03]Earth formed about 4.5 billion years ago. The timeline of our planet and the countdown [music] to this galactic collision almost match.

[00:03:11]When the two galaxies [music] finally meet, they won't smash like solid objects.

[00:03:16]Galaxies mostly [music] consist of empty space, and stars sit trillions of miles apart.

[00:03:22]During the merger, [music] most stars will slip past each other without direct impact, but gravity will twist [music] everything.

[00:03:29]Long tidal tails, huge streams of stars, will stretch out like glowing ribbons.

[00:03:33]Gas clouds will collide, spark furious waves of star formation, and light up the darkness.

[00:03:39]Over billions of years, the Milky Way and Andromeda will blend into one giant elliptical galaxy.

[00:03:46]A rounder, puffier system that astronomers sometimes nickname Milkomeda.

[00:03:51]All right, but what about us? The solar system will likely survive the chaos, but it won't stay untouched. Our position inside the galaxy could shift dramatically. We might move farther from the galactic center or swing into a new orbit entirely. [music] The night sky will transform.

[00:04:09]Andromeda already appears as a faint smudge about 2.5 million light-years away. Over the next few billion years, it [music] will grow larger and brighter until it dominates half the sky. Stars will scatter into new patterns.

[00:04:23]Constellations will lose their shapes.

[00:04:25]The sky your descendants see won't resemble the one above you tonight.

[00:04:30]So, for decades, scientists assumed Andromeda approached us simply because gravity between the two galaxies [music] overpowered expansion at short distances.

[00:04:41]That explanation worked, but it left gaps. Such as, why that exact speed and why that specific [music] trajectory.

[00:04:50]The new research shows the motion doesn't come from just two galaxies tugging [music] on each other. It comes from a whole invisible architecture.

[00:04:58]Dark matter sheets and expanding [music] voids shaping space like scaffolding behind a building.

[00:05:05]Dark matter itself still remains one of the biggest mysteries [music] in physics. It doesn't emit light and telescopes can't photograph it directly.

[00:05:13]Scientists [music] detect it by watching how galaxies rotate. Stars at the edges of galaxies move faster than visible matter alone could [music] allow.

[00:05:23]Something unseen adds extra gravity.

[00:05:25]That something makes up about 85% of all matter in the universe. Regular matter, that's everything you can touch, makes up [music] only about 15%. You live in the minority material.

[00:05:38]In fact, new research shows that even the supermassive black [music] hole at the heart of our Milky Way is a supermassive, but compact, clump of dark matter.

[00:05:48]That dark matter might be made of superlight [music] particles called fermions that can clump together into an extremely dense core surrounded by a wider, fuzzy halo.

[00:05:58]This structure could reproduce everything we've observed around Sagittarius A* the Milky Way's supermassive black hole.

[00:06:07]Everything from the crazy fast orbits of stars right near the center to the slower rotation of stars [music] further out in the Milky Way.

[00:06:15]So, this massive dark matter clump would put out the same gravitational [music] pull that we've been attributing to a black hole all this time.

[00:06:24]One of the most famous pieces of evidence for a black hole in our galaxy came in 2022 >> [music] >> when the Event Horizon Telescope captured an image that looked like a shadow, a dark circle surrounded by glowing gas right where Sagittarius [music] A* should be.

[00:06:42]But, new models suggest this dark matter core could cast a shadow that looks almost [music] identical to the one we saw in that image. It means our best visual [music] proof of a black hole might not be what we think it is after all.

[00:06:56]The study leans on data from the European Space Agency's Gaia mission, [music] which maps how stars move with astonishing precision.

[00:07:04]That data shows a kind of slowdown in orbital speeds far from the galaxy center called a Keplerian decline.

[00:07:11]The dark matter clump model can explain both this and the rapid motion near the center simultaneously, [music] something older models struggled with.

[00:07:20]Before we jump to conclusions though, scientists emphasize that this isn't proof [music] that the Milky Way's central object isn't a black hole.

[00:07:28]It's just that this [music] dark matter explanation seems to fit the observations perfectly well and might even do a better job explaining some features than the traditional idea.

[00:07:39]Upcoming observations, especially more detailed looks at the photon rings that light makes around black holes, could help tell [music] the two ideas apart.

[00:07:49]The idea that dark matter forms a vast, flattened structure around the local group reshapes how we [music] picture our cosmic neighborhood.

[00:07:58]Instead of a simple, roughly spherical halo, there's something more like [music] an enormous, invisible disc stretching between galaxies.

[00:08:06]Shape matters because gravity follows the distribution [music] of mass. A sphere tugs evenly from every direction and creates a balanced pull. A flattened sheet pulls more strongly along [music] its plane. That difference explains why Andromeda doesn't just drift [music] randomly. It slides along a gravitational track.

[00:08:28]But even as Andromeda approaches, [music] the universe keeps expanding.

[00:08:32]Distant galaxies continue to rush away from us faster and faster.

[00:08:36]In fact, billions of years from [music] now, our ancestors staring at the merged Milkromeda galaxy might not see any other galaxies at all. Expansion [music] will push them so far away that their light won't reach us.

[00:08:49]But here's a twist.

[00:08:51]>> [music] >> The same invisible dark matter sheet pulling Andromeda toward us also keeps our galaxy [music] intact. The same gravity that heads us to collision also built a structure [music] that lets stars form, planets grow, and life emerge.

[00:09:06]>> A vast galactic mystery has recently baffled astronomers. They've noticed that all but one of Andromeda's 37 satellite galaxies point toward the Milky Way. It's so extraordinary that it challenges current cosmology and might provide the answer to the dark matter question.

[00:09:24]So it happened like this. One day, scientists looked at those other satellite galaxies that go around Andromeda and spotted a really weird alignment. Now, we should keep in mind that Andromeda is kind of like our Milky Way's neighbor. It's huge, about twice as wide as the Milky Way, stretching 200,000 light years across. It also has way more stars, around a trillion, compared to our galaxy's mere 250 to 400 billion.

[00:09:53]Just like our galaxy, it has a bunch of tiny galaxies called dwarf galaxies spinning around it. There are 37 of them. The strange thing is, almost all of those little galaxies are on the same side of Andromeda. And it's getting even creepier because it's the side that faces the Milky Way. Only one galaxy is hanging out on the other side. Imagine tossing a bunch of marbles around a tree with nearly every single marble landing on just one side. That's weird, isn't it?

[00:10:24]Scientists say that this discovery doesn't match what they expected at all.

[00:10:28]Their usual space models show that stuff like this should be spread out more evenly. But this isn't even close to even. It's so lopsided that they can't really explain it yet. Those models follow the main space theory we use today called the Lambda cold dark matter model. According to this theory, the little galaxy should be spread around Andromeda pretty evenly, maybe just a bit lopsided. But in real life, their distribution is way more uneven than the model said it should be.

[00:10:58]The scientists have run tons of simulations, and only about 0.3% of them ended up close to what we actually see.

[00:11:06]And still, none of them match the real thing exactly. So, does it show the Lambda CDM model has flaws? That would be a big deal because we normally use this model to explain how the universe works.

[00:11:19]If the real sky doesn't match what our model says, especially in a place as close and well-known as Andromeda, it might mean that we're missing something big.

[00:11:30]Another strange thing, many of Andromeda's satellites seem to lie in a flat plane instead of being randomly distributed in all directions. This kind of flattened structure has been seen in other big galaxies, too, including our own, but it's still not well understood.

[00:11:47]Maybe this satellite disc is connected to the weird cosmic asymmetry, but no one really knows how.

[00:11:53]In any case, the fact that all the tiny galaxies on Andromeda's side line up almost perfectly in the direction of the Milky Way is super weird. It makes you wonder, could our galaxy be pulling or affecting them?

[00:12:07]But the catch is that nothing like this is happening to the Milky Way's own satellites. Plus, the gravity between our two galaxies doesn't seem strong enough to do that kind of pulling.

[00:12:19]The answer might lie in one of the most mysterious things the universe has to offer, dark matter.

[00:12:25]Let's start from the very beginning.

[00:12:27]We'll go all the way back to the early universe, just after the Big Bang. Now, back then, hey, I wasn't around then, but I'll tell you anyways, matter in the universe was spread out pretty evenly, but not perfectly even. In a few spots, there were just slightly higher amounts of matter than in others, tiny clumps.

[00:12:46]Those little differences were important because gravity could latch onto them.

[00:12:50]Over time, those denser regions started [music] pulling in more material, slowly growing into the first structures in the universe. Astronomers [music] believe that dwarf galaxies were the earliest and smallest building blocks of galaxy formation.

[00:13:04]Think of them like LEGO bricks, tiny pieces that were later pulled together by gravity to build larger galaxies over billions of years.

[00:13:13]Now, when we talk about matter, we mean two different things, normal matter, the kind we can see and touch, including atoms, gas, stars, planets, and everything around us, and dark matter, the invisible kind. It doesn't give off any light, doesn't absorb it, and doesn't interact with normal matter in the usual ways. The only reason we know it exists is because of its gravitational pull. There's also a lot more of it than normal matter.

[00:13:40]Scientists think that roughly 85% of all matter in the universe is dark matter.

[00:13:46]At the same time, we've never directly detected a dark matter particle. But we know dark matter is there because it bends light. It holds galaxies together.

[00:13:55]They spin too fast to stay intact otherwise, and it shapes the structure of the universe, allowing us to map its influence on larger scales.

[00:14:04]While normal matter can clump together, crash, heat up, cool down, and eventually form stars and galaxies, dark matter can't do any of that. It doesn't bump into itself or radiate heat. But it can still form big invisible clumps, which we call dark matter halos. In the early stages of the universe, the halos acted like gravity wells. They pulled in normal matter, which sank into the centers of those halos and formed the first galaxies.

[00:14:31]>> [music] >> So, galaxies like the Milky Way or Andromeda might be sitting inside enormous blobs of dark matter that we just can't see. So, cosmic asymmetry explained? Yeah, not yet. Scientists are sure that dwarf galaxies are some of the most dark matter dominated objects in the universe. In many cases, over 99% of their mass is likely to be dark matter.

[00:14:55]That means they give us a unique chance to study dark matter without all the messy gas and stars that exist in larger galaxies. Researchers use dwarf galaxies to figure out how dark matter clumps together or whether there are alternative theories of dark matter, like warm or self-interacting dark matter.

[00:15:13]At the moment, the main theory is that dark matter behaves like a cold, clumpy, and slow-moving fluid. At the same time, it might not be totally cold. Maybe it has other properties. Maybe it interacts with itself or with normal matter in subtle ways we haven't discovered yet.

[00:15:31]If that's true, it could help explain why satellite galaxies form in strange patterns like the ones around Andromeda.

[00:15:39]In other words, the Lambda CDM model might indeed have flaws. According to it, thousands of dwarf galaxies should be orbiting around larger galaxies like the Milky Way and Andromeda. But, we only see a few dozen. This is known as the missing satellites problem. There are other puzzles, too. For example, Lambda CDM predicts that dark matter halos should be the densest at the center, but many dwarf galaxies seem to have cores of their own. Plus, dwarf satellites around galaxies like Andromeda and the Milky Way seem to lie in thin co-rotating planes, and it's something extremely rare in Lambda CDM simulations. And finally, some simulated dark matter halos are so massive that they should form bright dwarf galaxies, but we don't see them.

[00:16:28]No wonder astronomers are so interested in dwarf galaxies and the explanation of cosmic asymmetry. For that, we need more precise measurements of their motions and structures and simulations that add more realistic physics like stellar feedback, turbulence, and so on.

[00:16:44]Plus, we got to search for new dwarf galaxies in the local group and beyond using better telescopes and deeper surveys.

[00:16:52]Now, getting back to Andromeda's weirdly positioned satellites, if the dark matter theory doesn't work out, there's another possibility that involves large-scale interactions between galaxies.

[00:17:03]>> [music] >> It could be an old collision or near miss that shaped the current distribution of satellites. For example, some scientists think a smaller galaxy, Messier 32, might have collided with Andromeda long ago, stirring things up.

[00:17:18]Scientists have seen some lopsided shapes in other galaxies before, but none as weird as what's going on around Andromeda. Usually, the little galaxies around big ones spread out pretty evenly or are just a bit off. That makes scientists think Andromeda might be a super special case. They say we need to keep looking using better telescopes to crack the Andromeda galaxy mystery. And perhaps, after figuring out the truth, we'll also come closer to solving the dark matter problem.