This was one of the leading theories for what dark matter is for quite some time. The term is "MACHO" or Massive Compact Halo Object. MACHO doesn't include just

This was one of the leading theories for what dark matter is for quite some time. The term is "MACHO" or Massive Compact Halo Object. MACHO doesn't include just black holes, but also a sea of objects small enough to be very difficult to observe but massive enough so that in aggregate they can cause what we observe as dark matter. However, the evidence against MACHOs is pretty strong. Once we had a theory of what to look for, experiments were designed to detect them. The thing we looked for is microlensing: Focus a telescope at some stars, and watch for a sudden, short jump in brightness. This can be caused by a massive object passing by in front of the objects, gravitationally lensing the light. We performed these studies and the number of microlensing events that we found were not sufficient to explain dark matter. So MACHOs as an explanation for dark matter isn't the answer. It's a good suggestion though, and like I said it was a leading theory for a while until we were able to test it. https://en.wikipedia.org/wiki/Massive_compact_halo_object https://en.wikipedia.org/wiki/Massive_compact_halo_object