What is a White Hole?
A white hole is the time reversal of a black hole: a region of spacetime that nothing can enter, and which continuously expels matter and light. It is not a speculation bolted onto relativity — it falls straight out of the same equations, because those equations do not care which way time runs.
Take the Schwarzschild solution, flip the sign of the time coordinate, and you get a second perfectly valid solution. Its exterior geometry is identical to a black hole's, which means it bends light in exactly the same way: the lensing in this scene is produced by the same integrator with the same acceleration law used everywhere else in this section. What changes is the ending. Where a black hole's marcher terminates a ray in darkness, this one terminates it in light. That single swapped condition is the entire difference between the darkest object in the universe and one of the brightest.
The reason it is almost certainly not out there is not that the mathematics is wrong but that the mathematics is not the whole story. Black holes form: a star runs out of fuel and collapses. A white hole cannot form from anything, because forming would be the time-reverse of evaporating. It could only have existed from the beginning of time, already present in the initial conditions of the universe. And it would not last — infalling radiation piles up on the outgoing flow, and the pile-up is thought to seal the thing off almost immediately.
They keep resurfacing anyway, because they are the cleanest illustration of something genuinely strange about physics. Every fundamental law we have is time-symmetric, and yet nothing in experience is. Eggs do not unbreak, heat does not flow from cold to hot, and black holes do not run backwards. The asymmetry lives in the initial conditions and in the second law of thermodynamics, not in the equations — and a white hole is what the equations look like when you take that fact seriously.