White holes

There are certain solutions of the equations of general relativity
which allow for the existence not only of black holes but of
objects called white holes too. But it turns out that this would be
possible only if the Universe had certain properties, called initial
conditions, at the time of the Big Bang. These highly speculative
objects are the opposites of black holes which, rather than sucking matter in, would spew matter and energy out into the Universe.
Their singularities would be different too and would mark the
beginning rather than the end of time. Unlike black holes though
there has been no conclusive evidence that white holes actually
exist. One of the problems is that the matter leaving them might
fall back in and the white hole would quickly be converted into a
black hole.
It is now believed that large white holes do not exist in
the Universe, but the rules of quantum mechanics regarding
particles and antiparticles suggest that if mini black holes
exist on the subatomic scale, then so should their antimatter
partners: mini white holes. Hawking has suggested that, just
as particle/antiparticle pairs can pop into existence for a fleeting
moment, so it should be possible for a black hole/white hole pair
to suddenly appear from nowhere. But don’t worry, they would
be too tiny to have any effect on us.