A German physicist and astronomer named Karl Schwarzschild proposed the modern version of a black hole in 1915 after coming up with an exact solution to Einstein’s approximations of general relativity. In a black hole, the 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.
Black holes are completely characterised by only three parameters: mass, rotation and charge. The black hole to Earth we know of is V616 Monocerotis, also known as V616 Mon. It’s located about 3,000 light years away, and has between 9-13 times the mass of the Sun.