If white holes are a thing, and the big bang is an example of a white hole, then I'm not sure how a white hole and a black hole could exist in the same space in order for them to collide?

If all of the matter in the universe came from a white hole, then prior to this there would be no matter to form the density required to create a black hole. And surely if there were any such matter then the mass of the 'white hole' would supercede that of the black hole and as such would have already swallowed it up.

Not claiming to have any great knowledge on this stuff, by the way. Just thinking out loud...