Q: How can we detect black holes?
A: There are two basic ways we can detect whether there's a black hole. Black holes are, of course, dark by definition - not even light escapes them!
The first way we detect black holes is by their gravitational influence. For example, at the center of the Milky Way , we see an empty spot where all of the stars are circling around as if they were orbiting a really dense mass. That's where the black hole is.
The second way is by observing the matter falling into the black hole. As matter falls in, it settles in a disk around the black hole that can get very hot. Some of the energy liberated from falling in is turned into light, which we can then see, for example, in X-rays.