Dark matter is basically this invisible stuff that doesn't reflect light or anything, so you can't even see it. Unlike regular stuff like stars or, I don't know, people, it totally ignores light, which makes telescopes useless. But it still has mass, so it’s pulling on stuff in space with gravity. Scientists only know it's there because they see stars moving way faster than they should. Plus, light bends weirdly around nothing, which they call 'gravitational lensing.' Basically, there's a ton of invisible junk pulling on everything. Researchers are guessing it might be 'WIMPs' or 'axions' or whatever, but nobody actually knows. It's like 85% of the universe and holds galaxies together, which is actually insane.