They are a thing, and are very much a part of modern cosmology and astronomy. We have the ability to detect distortions of space and time, and have even imaged the halo of black holes.

Carl Sagan, in the original Cosmos TV series, had a sequence of pictures from the Lewis Carroll books about Alice in Wonderland. Ah, found the clip. That might help you understand the idea: If you had enough mass, the pull of gravity could exceed the ability of light to escape it. So you get a black hole -- black because visible light doesn't escape.