Are there really black holes in space?
Since black holes do not put out any light, we cannot see them. However, we can see the effect that they have on the area of space around them. Since black holes have extremely high gravity, they pull in surrounding material at very high speeds, causing this material to become very hot and emit X-rays. By finding this very hot material which is spiraling into black holes, astronomers can identify where some of them are. Also, astronomers study the motions of objects in space to see where there is material that might be moving as if a black hole were affecting it. So far, evidence has been found for the existence of very massive black holes in the centers of several large galaxies, and much less massive black holes in binary star systems (where two stars orbit each other).