Astronomers are on a scavenger hunt  that started almost 100 years ago, searching for something  right in front of their eyes.

They know something’s out there, holding galaxies together and sculpting the observable universe into this awesome shape. And they’ve given it a name: dark matter.

The only problem is…no one has  any idea what dark matter is. It barely interacts with  anything except through gravity. It’s invisible, and all attempts  to detect it directly have failed.

But maybe… just maybe, a  giant moon orbiting Jupiter has been collecting evidence  of dark matter all along. [♪ INTRO] The problem with looking for dark matter is that we don’t even really know what we’re looking for. It’s like trying to go on an Easter egg  hunt if you’ve never seen an egg before. Also, the eggs are invisible.

And impossible to touch. But over the past several decades,  scientists have come up with all sorts of ideas about what  dark matter might be like. These days, one particularly popular hypothesis  is that it’s a bunch of tiny particles known as WIMPs that barely ever  interact with light and regular matter.

But very, very occasionally, a  WIMP could collide with an atom and release a little jolt of energy. Assuming they actually exist,  and this actually happens, we could detect those jolts. So around the world, scientists have built massive underground detectors full  of heavy atoms like xenon.

But after years of searching, they’ve had no luck. And that’s possibly because they’re  looking for the wrong thing entirely. Some physicists have proposed  that dark matter is actually made up of more massive clumps of stuff,  which they call macroscopic dark matter.

Unlike WIMPs, this stuff is supposed to be able to interact with regular matter, even violently. But these clumps would also be incredibly rare. Like, one might hit the Earth every 100,000 years.

That’s a thousand times  longer than the time humanity has even been thinking about dark matter,  let alone actively hunting for it. And in-between those hypothetical collisions, scientists have used process of elimination to figure out what we might be looking  for, and how we might find it. For one thing, we know roughly how  massive a typical clump would need to be.

And by “roughly”, I mean if  it were less than an apple, or more than a large asteroid,  scientists would have already detected macroscopic dark  matter in past experiments. That leaves a pretty big range in the  middle, but it’s better than nothing! And in a 2025 paper, one scientist  came up with a new plan to look for it.

The paper is still a preprint, meaning it hasn’t been peer-reviewed yet,  but this is the basic idea. If macroscopic dark matter is real,  surely it’s collided with objects in our solar system a bunch of  times in the last 4.6 billion years. Earth’s surface changes too much  to hold onto evidence like that.

But there are objects in our  solar system that barely change, recording every strike to  their surface across the eons. And we already have two space  missions headed to one of them. Ganymede isn’t just Jupiter’s biggest moon, it’s the biggest moon in the solar system.

In fact, it’s bigger than the planet Mercury. And its surface is old. A third of it is nearly as old  as the solar system itself, and the rest is thought to be  around two billion years old.

So Ganymede is covered in the scars of  everything that’s ever hit it in that time, including… just maybe… macroscopic dark matter. And the author of this 2025 preprint  realized that if macroscopic dark matter does exist, it would leave  a really distinctive scar. But before we get into more detail, we’ve got to keep the lights on with this  completely unrelated-to-dark-matter ad: Merci, Babbel for supporting this SciShow video!

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So feel free to click the link in the description or scan the QR code for 55%  off your Babbel subscription! When a typical asteroid hits  a rocky body like Ganymede, it usually shatters and leaves  behind a shallow crater. But according to models, macroscopic dark matter would have to be enormously/ dense.

I’m talking a billion times denser than water. The only place you can find stuff that dense in the known universe is in  the cores of collapsed stars. But that’s not the only extreme  property this stuff has got.

The macroscopic dark matter streaming through our solar system probably wouldn’t be local. It’d be sailing in from  somewhere else in the galaxy, traveling as fast as 270 kilometers per second. For comparison, your typical asteroid would crash into Earth going about 18 kilometers per second.

So a clump of macroscopic dark matter wouldn’t just burst into pieces when it  hit the surface of Ganymede. It would punch into the moon like a bullet… and maybe even come out the other side!!! So what kind of evidence would this  cosmic hit and run leave behind?

Our 2025 paper gives us an idea by simulating an impact from a 2-meter-wide dark matter bullet. Based on the exact numbers the author used, the impactor would bore a hole  roughly 5300 kilometers deep. But Ganymede’s average diameter  is just a bit smaller than that, hence the potential for an exit wound.

But Ganymede is also chock full of  ice, both on and beneath its surface. So while the dark matter blob  blazes through the moon’s insides, it vaporizes the ice around it and sends a shock wave racing outward from the borehole. Thanks to some thermodynamic happenings  we don’t have time to get into today, this winds up ejecting material  in the impactor’s path, which we will be coming back to, later.

Next, pressure drops in the borehole  as energy from the shock wave radiates through the moon for a couple seconds, until it loses enough energy and peters out. It leaves behind a massive cavity, which at its widest measures  a kilometer and a half across! But the destruction isn’t over.

Almost immediately, debris starts falling inward, starting wherever the pressure is  highest and the hole is narrowest. But instead of just refilling the  cavity, some of the infalling debris also gets blasted upward in  what’s called a Worthington jet. You know what happens when someone  cannonballs into a pool and a plume of water shoots up above them?

It’s basically the same thing as that. By the end, the impact event  leaves behind not just a crater, but a bunch of debris from deep  underground scattered in and around it. If the composition of that debris  differs from what Ganymede is normally rocking on its surface, astronomers  might be able to distinguish normal impact craters from those  caused by macroscopic dark matter.

And if they get really lucky, they might  even be able to match these craters with exit wounds, where the dark  matter flew out the other side. Now before anyone gets too  hyped about this hypothesis, we can’t do any of this crater searching just yet. It’s not a job for a regular telescope to tackle.

But in the early 2030s,  NASA’s Europa Clipper and the European Space Agency’s Juice,  will both reach Jupiter. And both will give us a clear  look at Ganymede’s surface at some point during their mission. If dark matter scars do exist,  the probes should spot them.

With all that said, we still  don’t have any amazing evidence that even suggests dark  matter really is macroscopic, let alone that it left scars on Ganymede. But some of our most important  theories started out as wild ideas. And when we’re dealing with one of  the biggest mysteries in science, sometimes it takes a wild idea to get somewhere.

Or at the very least, tell us where not to look. [♪ OUTRO]