A white hole is a black hole in reverse. Instead of absorbing all matter, energy and light outside of it as a black hole does, a white hole emits matter, energy and light, and cannot be entered from the outside. Once thought to be an impossible cosmological object, Carlo Rovelli and others have brought white holes back into contention. Alon Retter reveals the mistake physicists often make about white holes, and argues white holes play a vital role in the origin of the universe.

A black hole is a star with such a strong gravitational pull that light rays cannot escape from it. Scientists believe that in a black hole the matter is concentrated in a single point with infinite density (a singularity), or that it is impossible to know what happens within a small radius around its center - beyond the event horizon. In the last decades, very strong evidence has been accumulated for the existence of black holes in binary stars and the accepted concept is that at the center of most or perhaps all galaxies, including our own, the Milky Way, there is a black hole of huge mass – several hundred million times solar mass.