White holes are objects that only show up in physical theories - none have yet been observed. When a black hole is formed, all the matter and energy that falls into it should go somewhere. The idea is that somewhere else, either in our own universe or another one, another "hole" is created, the opposite of a black hole. This is why it's called a white hole. It's theorized that matter and energy that falls into a black hole comes out of a white hole somewhere else.

This is only science fiction at this point, and it may not be correct at all. Steven Hawking discovered something called Hawking Radiation where, in addition to matter falling into black holes, energy is also emitting as the black hole "dies". This may account for all of the "lost" energy.

It is not known whether white holes even exist, let alone how they might be formed.

One of the theoretical consequences of general relativity is the existence of "closed singularities", points where space (and matter contained in that space) is sucked into an infinitely small point where time comes to an end. These objects are known as black holes. Mathematically, the opposite situation can be envisioned, where space (and matter) is spewed forth from an infinitely small point where time begins. Again, for symmetry reasons, these hypothetical objects are referred to as "white holes". Because gravity attracts, rather than repels, it is easy to see how black holes would form, but not a white hole. The only "white hole" in existence that ever fits that description is the Big Bang, the even from which all space, time, matter, and energy originated, thereby creating the universe. Why there should have been a Big Bang, or any universe at all, is a question that science cannot answer yet.