The same scientific theory that predicted the existence of black holes also predicts the existence of white holes, the opposite of black holes in almost every respect.
While black holes are tough to spot due to their lack of emissions, white holes should be bright fountains of radiation and, theoretically at least, should be difficult to miss. Yet, so far, astronomers haven't been able to find any.
Very simply, a white hole could be considered a black hole that runs backward in time. That means there is actually no physical process in the universe that we know of that could create a white hole.
The seeming lack of white holes in our universe could mean that if a multiverse of universes exists, there could be a universe populated entirely by white holes with a complete absence of black holes.
That could be because time is a one-way system in each universe in the multiverse. In our universe, time can only run forward—the future is infinite—and that forbids the creation of a white hole. Meanwhile, in our multiverse counterpart, time can only play backward—the past stretches into infinity—and that forbids the existence of black holes while allowing white holes.
Some theoretical physicists also consider that maybe our universe had just one white hole, a tremendous one, right at its very beginning.
The Big Bang certainly sounds a little like a time-reversed black hole, if you consider the rapid expansion of space as being like a sudden eruption of matter at the beginning of the universe. And the forward-flow-of-time rule wouldn't apply if that white hole was here at the very start, before time started rolling forward.
Perhaps every universe in the multiverse starts with matter that flows out of a parent universe through a black hole connected to a white hole via a wormhole, a tunnel through spacetime between the black hole's entrance and the white hole's exit.
Whatever the case, since black holes have an event horizon that prevents accessing the information sealed behind it, we'll likely never be able to see into another universe through a white hole even if the two are linked.
Whether they are doorways to other regions of space or other universes entirely, or sealed exits, scientists are not set to stop speculating about white holes any time soon, meaning they will always be a doorway to the imagination.
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