Reshared post from +NASA
A black hole is a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen 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 special tools can see how stars that are very close to black holes act differently than other stars.
This image is an artist's drawing of a black hole named Cygnus X-1. It formed when a large star caved in. This black hole pulls matter from blue star beside it.
Image Credit: NASA/CXC/M.Weiss