As seen on Cosmos: A black hole is a place in space where gravity pulls so much that even light cannot get out. The gravity is so robust because matter has been squeezed into a little space. This can occur when a star is dying. Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The distinctive tools can see how stars that are very adjacent to black holes act in a different way than other stars. Seen here is an artist's drawing of a black hole named Cygnus X-1. It shaped when a large star caved in, and this black hole pulls matter from blue star beside it.
Image Credit: NASA/CXC/M.Weiss