Updated: Nov 4
Author: Shiven Arora
White holes are a figment in the realm of the mathematics that appear to be an impossible possibility. In simple terms, they are the hypothetical opposite of a black hole – literally, mathematically and physically. There is no concrete physical evidence for their existence, but they are mathematically possible.
Before developing an understanding of white holes, you need to understand what black holes are like. Black holes are regions of spacetime where there is a huge amount of mass packed into an infinitely dense point known as a singularity. To create this singularity, the black hole warps the fabric of space-time infinitely which causes the gravitational pull of the black hole to be infinite. As a result, the laws of physics cease to operate within the black hole’s event horizon. Past the even horizon, nothing can escape the black hole (including light) due to the strong gravitational pull. No one truly knows what happens inside a black hole because the light from inside can’t reach us. Hence, we have to rely on theories and equations.
Conversely, white holes only spew matter out from its own singularity of infinite density back into the universe. It is a region of outward flowing space-time with its event horizon prohibiting entry. Light cannot enter white hole but can only be radiated from the white hole due to the repulsive force. Both white holes and black holes have a mass so large that they warp the fabric of space-time to such an extent that an object can only travel in one direction. Physicists describe a white hole as a black hole's "time reversal", a video of a black hole played backwards. 
Figure 1: White hole.
Mathematics allows for white holes to exist because there are two solutions to the equations for Einstein’s general theory of relativity. One solution confirms a black hole. However, the second solution produces a white hole. There are two solutions to Einstein’s equation because the direction of time takes no preference in general relativity (time symmetric). As a result, both solutions are equally likely to exist according to mathematics. Neil De Grasse Tyson (a theoretical physicist) related this mathematical phenomenon of two solutions to the answer to the 'square root of the number 9'. In this case you also produce two answers – positive 3 and negative 3. Neither answer is better or worse than the other and are both correct. Likewise, in Einstein’s equations, black holes and white holes are equally correct solutions mathematically.
However, if white holes arise from the collapsing of stars, you are left with a massless singularity. Physicists debate theories over how a white hole can spew matter with actual mass from a massless singularity. It doesn’t seem to make much sense. However, some believe that a black hole can only shrink until hitting a natural limit. That’s when it would rebound outward due to an immense amount of outward pressure in a “quantum bounce.” This would turn the shrinking black hole into an expanding white hole and eject all the mass the black hole had sucked in beforehand all at once. It would not be able to last very long due to the instability of white holes. 
This theory using quantum mechanics solves many issues that would arise if only black holes existed out of the two. Firstly, it solves the ‘black hole information paradox’ which is related to the laws of thermodynamics. Once matter is sucked into the black hole, no one knows what happens to the information of the matter and it appears to be deleted from the universe. As per the laws of thermodynamics, that this cannot occur because information can never be destroyed from spacetime. Einstein’s theory of general relativity and quantum mechanics also rely on the laws of thermodynamics being true. Whatever comes out of a white hole would be a mangled version of the matter which went into the black hole, but information of its former self would not be eradicated. 
Another thinking that holds onto the idea of information being retained is that a white hole sits on the other end of a black hole connected by a wormhole (theoretical tunnels of spacetime). In this way, the information of matter going into the black hole is retained and transferred to another universe.
Figure 2: Einstein-Rosen bridge. [https://jila.colorado.edu/~ajsh/bh/schww.html.]
The image above is a complete Schwarzschild geometry representing a white hole and black hole connected by a wormhole. This wormhole connecting two separate universes is called the "Einstein-Rosen Bridge". 
Some physicists associate the idea of white holes with the Big Bang due to the instability of them both. The Big Bang's explosion of matter and energy looks like potential white hole behavior.  The formation of our universe could potentially be the result of a white hole spewing out all the matter we observe in our universe.
It’s alluring to our sense that things regularly come in binaries — if there’s an off switch, there’s undoubtedly an on switch elsewhere. White holes feel like a required balance to the conclusiveness of black holes: where does all of that sucked-up stuff go? 
However, physics has its limitations with the idea of white holes. They violate the second law of thermodynamics by decreasing the entropy of a system which would make them impossible to exist. This makes many physicists skeptical of these mathematical possibilities. Alternatively, they do provide answers for certain unanswered questions left by black holes alone. The only potential source of evidence of white holes was a gamma ray burst from outer space in 2006. It lasted 102 seconds and was accompanied by an explosion of white hot light coming from nowhere which immediately vanished. This explosion left physicists confused because most gamma ray bursts only last a couple of seconds from supernovae and neutron stars. However, some believe that this was maybe a white hole and could lead to the secrets of our universe…
 Charlie Wood. https://www.space.com/white-holes.html.
 Andrew Hamilton. https://jila.colorado.edu/~ajsh/bh/schww.html.