A black hole is a region of spacetime where matter has collapsed so densely that gravity lets nothing escape — not even light can get out past its boundary, the event horizon.
Black holes follow from Einstein's general relativity: Karl Schwarzschild published the solution in 1916. Einstein himself doubted that they could exist in nature (1939). In 2019 the Event Horizon Telescope published the first image of one, in the galaxy M87; in 2022 it followed with Sagittarius A* at the centre of our Milky Way.
Source: Wikipedia: Black hole — Read the full article on Wikipedia.