We often find a black hole because other things we can see are orbiting a patch of nothing. Based on the orbits we can calculate how heavy the thing they should be orbiting is.
A black hole that doesn’t have anything visibly in orbit around it would be much harder to find, since it’d just appear to be a patch of nothing with nothing around it–which describes most of outer space to begin with. Though we could still spot one based on how it bends the light coming from things roughly behind it.
We often find a black hole because other things we can see are orbiting a patch of nothing. Based on the orbits we can calculate how heavy the thing they should be orbiting is.
A black hole that doesn’t have anything visibly in orbit around it would be much harder to find, since it’d just appear to be a patch of nothing with nothing around it–which describes most of outer space to begin with. Though we could still spot one based on how it bends the light coming from things roughly behind it.