system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know that the concept of dark matter isn't just the observed gravitational effect of undetected black holes on their surrounding stars and galaxies?
gpt
To get the correct rotation curves for galaxies, you would need to have large numbers of small black holes distributed in a roughly spherical-shaped halo around a galaxy. Small numbers of big black holes would stir things up and we'd notice that. It needs to be a fairly smooth distribution of matter.

Given there must be a lot of these black holes - roughly as many as there are stars in the galaxy - then quite often one of them will just happen to pass between us and a distant star. When this happens, the black hole bends the light of the star towards us, and the star gets a little brighter. This is caused *gravitational microlensing*. It also happens to a smaller extent just when any star passes in front of any other.

We have scanned large chunks of the sky for long periods of time to detect these microlensing events. Based on the pattern of how the background star gets brighter, we can learn about what sort of object is doing the lensing. And we have found a number of microlensing events. But what we have found doesn't give any evidence that there's an extra population of black holes (or dim brown dwarfs, which would work too) out there - it's totally consistent with the number of visible stars that we see.