Astrophysicists have no idea what dark matter is (though they have a few theories), and it’s impossible to directly observe. Given those two facts, it’s reasonable to ask why we think it exists at all. Medium’s Amanda Yoho lists five good reasons, all centering on one factor: gravity
Simply put, the universe acts gravitationally as if it’s mostly made up of stuff we can’t see, and since we can’t see this stuff, we call it dark matter. It’s no worse (and more accurate) than talking about “empty space” or “uncharted territory,” and it’s also no more specific; dark matter could turn out to all be made out of one thing, or it could turn out to be even more diverse and complex than the parts of the universe we can see. We don’t know, and we have no way of knowing.
What we do know is that either undiscovered matter exists, or our entire understanding of gravity needs to be revised in some way. And as Yoho explains, revising our entire understanding of gravity would create more problems than it solves:
We have fantastically precise measurements of gravity’s influence on objects throughout our solar system which fit precisely within the current understanding of gravity from General Relativity (a fact that underpins the precision of modern GPS). If you want to change the theory of gravity, you have to preserve its behavior as we’ve already measured it in the solar system.
If we assume that dark matter is more-or-less homogenous, the best candidate seems to be sterile neutrinos. But the truth of the matter is that we know pretty much nothing about dark matter—that’s what makes it dark—and there could easily be entire secret universes hidden within our own, just out of reach. We really don’t know yet, and that’s tremendously exciting.