[intro].

We’ve known for decades that all the stars, galaxies, and other stuff we can see in space make up less than 20% of the matter in the universe. The rest of it is dark matter.

It’s an invisible something we can only detect as its gravity pulls on the regular matter around it. But we know it’s out there, holding together galaxies and giving the universe its structure. And since regular matter is organized into stars and galaxies, that kind of makes you wonder… could dark matter do the same?

Could there be dark matter stars? Or a dark matter galaxy? These are still open questions.

But by exploring them, astronomers can learn more about the nature of dark matter. And that can offer us clues about how hidden forces shape the visible and invisible universe. What makes this such a hard question is that we don’t know what dark matter is like.

Many scientists assume it’s some kind of particle. But all we really know is that there’s a mysterious source of gravity in galaxies, and we can’t see it. It doesn’t absorb, emit, or reflect any kind of light, or radiation.

It’s just dark. And actually, that makes it pretty unlikely that dark matter could ever form a star. See, normally, a star begins forming when a cloud of gas collapses in on itself.

But for that to happen, the atoms in the gas first have to cool down. As they do, they vibrate less violently, which lets them pack together tighter and tighter, until there’s enough pressure on the atoms’ cores to kick off nuclear fusion. The problem with dark matter is that it doesn’t absorb or emit radiation, so hypothetical dark matter particles can’t heat up or cool down.

And if they can’t do that, scientists don’t know if they could ever get dense enough to form a star. Now, to be clear, there is a lot we don’t know about dark matter. So say that, somehow, it could get around this cooling problem.

Well, in that case, some researchers think a dark star wouldn’t be completely out of the question. In 2007, one team even modeled how it might work. They started with a theoretical dark matter particle called a neutralino.

According to hypotheses about dark matter, when two neutralinos come together, they annihilate each other and release a burst of high-energy radiation. And according to the team’s model, this process could make something vaguely star-like. The idea is that if some neutralinos came together in a cloud of regular gas and started annihilating each other, the gas around them would absorb the radiation and heat up.

The cloud wouldn’t shine like a normal star. Instead, the heat would make it glow in infrared light. The cloud wouldn’t be nice and round and compact, either.

But if a dark star existed, this team thinks this is probably how it would work. Also, even though this is still hypothetical, the nice thing is that it does give us an idea what to look for. If neutralinos do exist, theoretical work suggests that they should kick out things like gamma rays and neutrinos when they annihilate each other.

So if we saw a bunch of those within an interstellar gas cloud, it could be a sign of a dark star. That said... dark stars are still a long shot. But dark galaxies are much more likely.

That’s because dark matter already makes up some 90% of your average galaxy, forming a halo around all the stars and gas. And the key is, astronomers think this dark matter halo is actually what forms first. Its gravity draws in more and more gas, which eventually becomes dense enough to collapse and ignite stars.

But there’s evidence that this doesn’t always happen. For instance, in 2005, astronomers detected an immense cloud of gas that had no stars, but was still weirdly massive. Like, so massive that it warped the space around it and distorted the light from nearby galaxies.

Based on measurements of the cloud’s rotation, the team calculated that the culprit was a bunch of dark matter. And by “a bunch,”. I mean this cloud had 100 times more dark matter than the normal stuff.

So, it seems like this thing got as far as the dark matter halo stage, but for some reason never formed stars like normal galaxies. One possible explanation is that there just wasn’t enough gas nearby for the halo to draw in, so the cloud couldn’t get dense enough to make stars. Either way, this doesn’t quite qualify as a true dark matter galaxy, because it contained a whole lot of regular matter.

But the fact that this cloud is out there suggests that an entirely dark galaxy isn’t so far-fetched. To find one, we might look for a huge, invisible area in space that’s warping light: a sign that something with a lot of mass is hiding just out of reach. Either way, whatever we find or don’t find can help us understand what dark matter is like— and how it’s connected to all the stuff we do see.

And as we piece those things together, we can get a fuller picture of how our universe took the shape it has today. So, every month, the SciShow team makes a new, exclusive pin. And this month, we got a little extra creative: The pin is our interpretation of a dark matter star.

I know. We couldn’t help ourselves. If you want one, they’ll be available until the end of March, and you can find them at DFTBA.com/SciShow. [ outro ].