Could dark matter just be hidden planets?

Astrophysicist Katie Mack had this answer for us...

Katie - So dark matter is a kind of matter that we can't see. When it was first discovered to be something that's out there in the universe, it was discovered through the way that it affects the rotation of galaxies. So we live in the Milky Way galaxy, it's a sort of disc shape, with a lot more stars and stuff at the centre, and then it sort of flattens out. And the stars orbit around the centre of the galaxy; it takes millions and millions of years. And you can tell by looking at a lot of different galaxies that the stars go around them a lot faster than you would think they should. If you look at the galaxy and you count up all the light in the center of the galaxy and all the light from the stars and the gas and the dust, and even assume that there's a supermassive black hole in the centre, the stars are still going more quickly around the outer edges than they should. You would expect that they would just kind of fling off into space because there's just not enough gravity, from what you can see, to hold them in. And so based on that and a whole huge number of other things, we conclude that the galaxies are actually mostly made of something invisible, something that we can't see, some kind of matter that's providing the extra gravity to hold everything together. And for a long time, we didn't have any ideas for what that could be. It was just something that wasn't putting out light. But we've determined that it really can't be anything like planets or dust or atoms, regular stuff, because it doesn't absorb light. So if it were something like rogue planets, then it wouldn't be invisible, it would be opaque. And we don't see that. And we also don't see the dark matter sort of collapsing together into a disc the way that regular matter does. So in a similar way as how, if you had a lump of pizza dough and you spin it, everything in space, when it comes together with gravity, there's some overall spin and then it all averages out to that kind of discy shape, and dark matter doesn't seem to do that. We can see evidence that dark matter stays kind of puffy and rounded in a way that regular matter doesn't. So we have a number of reasons to suspect that it's not regular matter. It's not something like planets. It does seem to be something different, something that can bump into other dark matter and just pass right through and not collide and get stuck in the middle. And we have a few pieces of evidence for that as well, where we've seen entire clusters of galaxies collide with each other. And a lot of the regular matter gets stuck in the middle, but the bulk of the gravity, the dark matter, passes right through the collision. And we see it show up on the other side after these collisions. So we're pretty sure that there's something weirder than that, we think maybe it's some kind of new particle that we didn't know about before, that just doesn't interact in the ways that most other particles do. Specifically, doesn't do electromagnetism, which is light and magnetism and electrostatic repulsion and all of that. You know, like a neutrino, which is another kind of particle that doesn't seem to do that force. It doesn't do electromagnetism. It's probably some kind of new particle.