When we think of black holes and big celestial objects we think of objects being pulled towards them with a strong force. And that force turns out to be the gravitational force but then for the theory of whiteholes what force must there be that is responsible for the opposite of the scenario given above. Negative gravity? Negative mass?
White holes are time-reversed black holes. For a black hole, a particle may fall in with a trajectory that terminates at the singularity. The time-reversed version of that picture is that a particle "is created at the singularity with sufficient speed that it can escape" (roughly).
What causes the creation of such a particle with such "speed"? Established physical theories can't explain this any more than they can explain what happens when a particle hits the singularity of a black hole. So in this respect, the answer is "we don't know".
But I also want to emphasize that white holes are gravitationally attractive. Time-reversed gravity still acts in the same direction (imagine the time reversal of jumping into the air and landing). An outflowing particle is slowed as it escapes the white hole's gravitational pull.
(Note that I'm speaking of "white hole" solely as a solution to the equations of general relativity. There is no reason to think that white holes can or do exist in the universe.)
I think it's worth keeping in mind that a lot of the exotic theoretical objects related to black holes are things that show up purely as solutions to the Einstein field equations. The field equations specify how the curvature of spacetime has to correspond to the mass and energy that is present, and a solution generally takes the form of a formula specifying the metric, which tells how distances and angles are measured at any point, and from which the curvature can be calculated. Not every solution applies to something in the real world.
The black hole solution was, I believe, the first nontrivial solution to be found, and at first people doubted that actual black holes could exist. But the solution was still useful because it is still correct as long as you are outside a spherical body like the earth or the sun. Over time, theoretical evidence piled up that black holes really could exist, and observational evidence piled up that they really do exist.
White holes are different. Generally in physics, the basic laws are still true if you reverse the direction of time, and white holes are what happen if you do that. It's another valid solution to the equations. But it's not the sort of thing you would expect to exist, and we don't have any concrete evidence that one would exist. This doesn't mean very smart physicists haven't come up with ways they could be part of the universe, but the best guess is that they aren't.
I think wormholes are similar. There are mathematical solutions that have them, but there are not obvious natural process that could lead to them.
There are diagrams that can help you to visualize these solutions. One kind is called a Kruskal-Szekeres diagram, and another is a Penrose diagram. But they are not always so easy for non-experts to interpret, as the comments will indicate. You also might find this question helpful, if you haven't already seen it.
Gravity itself. White holes are a creation of gravity, just in a repulsive way. Gravity is defined as the curvature of spacetime, which can be negative too as the spacetime can be negatively curved in a saddle like shape, it produces repulsive force just like a mountain compared, similar to actions of exotic matter.
Black holes are defined via the Einstien field equations. However white holes are just the time reversal of black holes, so technically Einstien's field equations apply here, except that it is the time reversal. According to the CPT symmetry, especially the T symmetry, even if we reverse time itself, the law of physics would remain the same. So black hole or white hole, the Einstien field equation apply's to both. So the Riemann's metric tensor has only the values in negative, due to the reversal of time, everything else is the same.
In conclusion it can be defined as time reversed gravity.
Thank you, Hope it helps you!