What is a white hole?

A white hole is a mathematical construct. It is not something "real" like a black hole, which could be said to be the "generator" or "origin" of the white hole. The white hole arises as a result of the mathematics that describe what is happening inside a black hole.

Because of the immense gravity involved, a time component in the math is included (thanks to Big Al Einstein). This time component is an expression of "moving time" and speaks to the movement of time in the "forward" direction, like we normally understand it. But the equation will solve with time moving "backwards" just as well as the other way. This gives rise to some possibilities.

Let's get real here. We don't know for sure what's at the bottom of the gravity well of a black hole. Is it a dimensionless point called a singularity or is it a super-compressed ball of ultra-dense matter? We don't know. And with so much uncertainty there, how can we make the call as to whether or not a white hole is a "time machine" with time moving backward or something else? And if it ejects matter as some theories have suggested. Where does the matter come from? The black hole? There are more questions here than answers.

Come away from this with the idea that a white hole is a "gimmick" of sorts that results from some slick mathematics. More than that cannot be said. Note that some things were predicted by looking at mathematical expressions and then saying, "Hmm. If we "reverse the sign" here, the thing still works, and the description of "something" as expressed would have the opposite characteristics of the "real thing" as we know it. Later on, we find the "other thing" and say, "Ah ha!" Will the same thinking apply to the white hole? A white hole is the time reversal of a black hole, another singularity in space-time. Matter emerges unpredictably from a white hole (unlike a black hole, into which matter is drawn). An example of a white hole is the original singularity of the Big Bang.

White holes appear as part of the vacuum solution to the Einstein field equations describing a Schwarzschild wormhole. One end of this type of wormhole is a black hole, drawing in matter, and the other is a white hole, emitting matter. While this gives the impression that black holes in this universe may connect to white holes elsewhere, this turns out not to be the case for two reasons. First, Schwarzschild wormholes are unstable, disconnecting as soon as they form. Second, Schwarzschild wormholes are only a solution to the Einstein field equations in vacuum (when no matter interacts with the hole). Real black holes are formed by the collapse of stars. When the infalling stellar matter is added to a diagram of a black hole's history, it removes the part of the diagram corresponding to the white hole.

The existence of white holes that are not part of a wormhole is doubtful, as they appear to violate the second law of thermodynamics. Quasars and active galactic nuclei are observed to spew out jets of matter. This is now believed to be the result of polar jets formed when matter falls into supermassive black holes at the centers of these objects. Prior to this model, white holes emitting matter were one possible explanation proposed.

A more current view of white holes takes into consideration a revision to the standard model of the big bang theory which states that the big bang is an explosion that happens within a black hole, with the expansion that follows the traditional interpretation of the big bang, expanding into infinite space inside the black hole. Or in other words, a miniature universe is created at the core of the black hole, which expands into extra dimensions outside of this universe. The expansion taking place in this new miniature universe, if it could be perceived from an observer from this universe, could be looked at as a white hole. Matter that could not escape the intense gravitational pull of the black hole in this universe is instead sent speeding into the newly expanding baby universe.

Using that logic, one could assume that our universe itself is a white hole. Hypothetically, this model could be used to explain the increasing rate of expansion of this universe: as matter from our parent universe is engulfed by our parent black hole (the black hole that created our universe), our own universe is fed this matter which could possibly have something to do with dark matter and dark energy, which currently is thought to contribute to the increase in the rate of our universe's expansion.
Don't u mean black hole. Black holes can suck the whole universe. There are millions of them up in space lucky we are still alive. They say after 5 million years this may happen.