Not just black holes, but neutron stars and brown dwarfs, too. These studies ruled out MACHOs with masses in the range of 10^(−8) to 100 solar masses. That's

Not just black holes, but neutron stars and brown dwarfs, too. These studies ruled out MACHOs with masses in the range of 10^(−8) to 100 solar masses. That's 1/3rd the mass of the Moon to five times the mass of Betelgeuse. For black holes to be visible due to Hawking radiation against the cosmic microwave background, they would have to be pretty small, about 0.8% of the mass of Earth or about the mass of the Moon (https://arxiv.org/abs/astro-ph/9911309 https://arxiv.org/abs/astro-ph/9911309). I don't think anyone's found such a small black hole anywhere, yet. If they did, it would be a pretty big discovery since such a small black hole would have to be primordial. As an aside, "dense" doesn't really make sense in the context of a black hole, which has infinite density.