>We call it "dark matter" because our formulas give the right prediction if we add a lot of mass where we see none, that is, if we assume there is some matter there that does not emit or interact with light.
I'm not a physics expert so forgive me if this is a naive take, but could black holes just sitting around, too far from other objects to have a visible effect, account for this unaccounted mass? IIRC they're small in size but incredibly heavy, and if space is infinite then they could just be sitting around taking up mass.
Someone has thought of this, mainly involving primordial black holes. IIRC the idea is that right after the big bang, when the subtle density differences in the gas permeating the entire universe caused pockets of gas to contract and eventually form stars, some pockets would collapse into black holes directly.
edit: I should add that they can't be regular black holes formed from stars, because then they'd have the same distribution of the stars in the galaxy they formed. While the the stars in a galaxy are most numerous near the center and get less numerous away from the center, the mass we have to insert in the form of dark matter to make the equations add up has to be evenly spread from the center and far beyond the visible edge of the galaxy.
The problem with this primordial black hole idea is that having lots of such black holes about would leave a very tell-tale signature thanks to gravitational lensing, which would be very strong near the event horizon. So far, no searches have found this.
I think part of the issue with dark matter and similar is that the main stream mostly hears about the theories that are the best candidates, and not about all the alternative explanations physicists have thought about but discarded due to some more or less obvious issues. Most won't even make it to arXiv, since they'll be eliminated by some rudimentary cross checks.
So the public might get the impression that physicists don't even try to come up with alternatives, but that couldn't be further from the truth.
I'm not a physics expert so forgive me if this is a naive take, but could black holes just sitting around, too far from other objects to have a visible effect, account for this unaccounted mass? IIRC they're small in size but incredibly heavy, and if space is infinite then they could just be sitting around taking up mass.