White holes are theoretical depressions in space-time that form from black holes. In Einstein's theory of General Relativity, when a massive star dies, it collapses in on itself, extinguishing the nuclear flame. If the star is massive enough, there's so much gravitational force that it rips through space-time, like a bowling ball through a thin sheet of paper. A white hole is the reverse of the black hole that forms.
White holes exude the matter that the black hole progenitor sucked in. Instead of the event horizon keeping all matter and energy inside, as it does in a black hole, a white hole's event horizon cannot be entered. Because a white hole is the reverse of a black hole, it cannot be formed through gravitational collapse. It only exists as a solution within a pre-existing, eternal black hole in the equations of General Relativity.
This is because General Relativity makes no distinction between past and future, whereas, according to Stephen Hawking, the combination of quantum physics and General Relativity produces black holes that emit radiation on a very long timescale. Therefore, the incorporation of quantum mechanics into the solutions of General Relativity does acknowledge the existence of past and future and white holes are simply black holes in the distant future.