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.

With black holes, as viewed from outside, nothing ever passes through the event horizon. We just see time slow down for those things as they get infinitely closer until the end of time. (From the point of view of the infalling object, things are very different.) Reversing the process, for a white hole, objects would be observed to be moving outward as far back in time as you could see. Nothing pushed them outward -- the universe just happened to start out that way. That seems kind of random and unlikely, and we don't see examples, so it's not thought they are likely to exist.

To be fair, the big bang itself might seem like an example of that, but there is an initial "inflationary" phase that I'm pretty sure does not match up. (An expert would have to weigh in on this.)

I think wormholes are similar. There are mathematical solutions that have them, but there are not obvious natural process that could lead to them.

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.

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.

With black holes, as viewed from outside, nothing ever passes through the event horizon. We just see time slow down for those things as they get infinitely closer until the end of time. (From the point of view of the infalling object, things are very different.) Reversing the process, for a white hole, objects would be observed to be outside the event horizon, and moving outward, as far back in time as you could see. Nothing pushed them outward -- the universe just happened to start out with them moving outward. That seems kind of random and unlikely, and we don't see examples, so it's not thought they are likely to exist.

To be fair, the big bang itself might seem like an example of that, but there is an initial "inflationary" phase that I'm pretty sure does not match up. (An expert would have to weigh in on this.)

I think wormholes are similar. There are mathematical solutions that have them, but there are not obvious natural process that could lead to them.

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.

With black holes, as viewed from outside, nothing ever passes through the event horizon. We just see time slow down for those things as they get infinitely closer until the end of time. (From the point of view of the infalling object, things are very different.) Reversing the process, for a white hole, objects would be observed to be moving outward as far back in time as you could see. Nothing pushed them outward -- the universe just happened to start out that way. That seems kind of random and unlikely, and we don't see examples, so it's not thought they are likely to exist.

To be fair, the big bang itself might seem like an example of that, but there is an initial "inflationary" phase that I'm pretty sure does not match up. (An expert would have to weigh in on this.)

I think wormholes are similar. There are mathematical solutions that have them, but there are not obvious natural process that could lead to them.