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Black Holes of reasonable can be detected by gravitational lensing. If Black Holes / Brown Dwarfs / etc. were primarily responsible for the dark matter locally, lensing searches would have detected more. See e.g. https://arxiv.org/abs/astro-ph/0002058 for an overview.

So according to wikipedia [1] they have been looked into, they would fall under the category of MACHOs

1. https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter

They're detectable and already taken into account.

This graphic can explain very clearly, although it’s a little old. The result is that constraints today are even tighter though, so for these purposes it’s still useful.

https://4.bp.blogspot.com/-UN8jPwzj2qI/V9hRVCG1HBI/AAAAAAAAC...