A white hole is a time-reversed black hole – a region of space-time where matter spontaneously appears and explodes outwards, rather than implodes and disappears as with a black hole.
White holes are essentially the opposite of black holes, in that they spit out light and matter, rather than trapping it.
So far, white holes they are purely hypothetical objects, but astronomers are contemplating how they could form in reality.
A black hole forms when a massive star at the end of its life shrinks catastrophically under its own gravity down to an infinitesimally small point, or singularity.
All that is left behind from the stellar collapse is a grossly warped region of space, so where does the star’s matter actually go?
One possibility is that it explodes into another universe as a white hole.
The white hole has to be in another universe because the defining characteristic of a black hole is that the event horizon, which hides the singularity from view, is one-way.
In other words, things that fall through it can never emerge again in our Universe.
For matter to pass between universes, the black hole and the white hole must be connected by a wormhole – a tunnel through space-time.
Some scientists have suggested that super-energetic objects such as quasars are white holes, and perhaps even the Big Bang itself was a gigantic version.