There are any number of jokes to be made here. All of them are inappropriate. Suffice it to say that white holes are a possible product of black holes, and they might show us what other universes look like.
The idea that scientist could get a peek at matter from other universes – or other parts of the universe – would be wildly exciting if it weren't for one thing; no white hole has ever been observed. A NASA site calls white holes ‘very hypothetical.' Actually, it calls white holes ‘VERY hypothetical,' because, as far as I can gather, it is unacceptable for NASA to call something ‘probably complete bullshit,' especially on a kid-oriented page.
Basically, a black hole sucks in mass and collapses it to a single point. The idea of white holes takes black holes from a nearly mystical mathematic concept and turns them into a cosmic digestive tract. What goes in must come out. Black holes could, theoretically, punch through to another universe – or another part of a universe – and hork some matter all over that other universe's front lawn. The horking side of the black hole would be a white hole, because it gives off energy.
What makes these things so entirely hypothetical that some NASA scientist went capslock on their ass? Well, there is the fact that while we've seen plenty of black holes, so far we haven't seen any white holes excreting matter or energy like they've been stuffed full of bran muffins. That's not a good a sign. We also haven't observed a lot black holes' event horizons shrinking in a way that shows the black holes might be losing their matter off the side of a boat in rough weather.
So what keeps the dream alive? There's a lot of energy coming from the center of galaxies that can't be accounted for. Since mass and energy are equivalent, something might be upchucking energy, and it isn't anything we've seen yet. Could it be the mysterious white hole? Only time and more observation will tell.