Since 90 % of matter is what we cannot see, why can't it be black-holes from early on? Is is possible to figure out that there are no black holes in the line of sight of various stars/galaxies we observe?
Answer
Even quiescent black holes tend to show up, through microlensing. Observational tests have put pretty rigorous constraints on a range of black holes masses in the Milky Way, although intergalactic black holes are not as well constrained.
The other problem is figuring out how you make lots of black holes, especially at smaller scales. That's not to say that scientists aren't still coming up with ideas, though. Warning: shameless plug. Further warning: We never found any empirical confirmation, so until that happens, it was just a promising idea that didn't pan out.
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