As a layperson who watches way too many theoretical physics and astronomy documentaries, I've seen the folded-piece-of-paper analogy of wormholes a bajillion times. Usually they explain that black holes might connect to other parts of the universe, but they don't really explain what would have to be on the other end for any kind of information to travel through it. When they do, they usually suggest white holes -- but I'm not entirely convinced.
I don't have too much trouble understanding how a black hole forms, and to some extent, how that could lead to a distortion of space-time sufficient to causing the "fold". But it's hard to imagine what would cause a white hole to form. Even if there were such thing as negative mass, there doesn't seem to be any way that an object can come to have a negative density sufficient to becoming a white hole, so there couldn't possibly be a process that causes them to form.
So if black holes could be wormholes at all, it doesn't seem to be the case that the exit would require matter to be present before it forms. If anything, a black hole wormhole would have to "perforate" a white hole or some other kind of exit into existence -- somewhere.
But there doesn't seem to be anything that determines where that exit would form, so one could exist outside the boundaries of the matter that defines the edges of the universe. That would mean that the universe is infinite in size, since white holes could (and probably would, if their position were completely random) appear very, very far away.
So then either black holes can't be wormholes, the universe is infinite in size, or I'm missing something. Neither of the former two conclusions have been reached to the best of my knowledge -- so what am I missing?