In general relativity, a white hole is a region of spacetime which cannot be entered from the outside, but from which matter and light may escape. In this sense it is the reverse of a black hole, which can be entered from the outside, but from which nothing including light may escape. White holes appear in the theory of eternal black holes. In addition to a black hole region in the future, such a solution of the Einstein equations has a white hole region in its past. This region does however not exist for black holes that have formed through gravitational collapse, nor are there any known physical processes through which a white hole could be formed.
There are no eternal white hole solutions. White holes must either evaporate in a finite time through emission of matter or must be connected to a future black hole region through an Einstein-Rosen bridge.
Like black holes, white holes have properties like mass, charge, and angular momentum. They attract matter like any other mass, but evaporate or form a black hole before any attracted matter can reach it.
In quantum mechanics, the black hole emits Hawking radiation, and so can come to thermal equilibrium with a gas of radiation. Since a thermal equilibrium state is time reversal invariant, Stephen Hawking argued that the time reverse of a black hole in thermal equilibrium is again a black hole in thermal equilibrium.
This implies that black holes and white holes are the same object. The Hawking radiation from an ordinary black hole is then identified with the white hole emission. Hawking’s semi-classical argument is reproduced in a quantum mechanical AdS/CFT treatment where a black hole in anti-de Sitter space is described by a thermal gas in a gauge theory, whose time reversal is the same as itself.