A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.
A black hole is found by its interaction with matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a black hole caused by a companion star or nebula, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation. This radiation can be detected from earthbound and Earth-orbiting telescopes.