They just do, not much explanation required. They are actually making some miniature black holes in a supercollider. This will most likely create a black hole that will suck up the earth. However, if that doesn’t happen, we should use them in vacuum cleaners. If this thing can suck up light, there is no chance for the dirt in my carpet. The very second that Dyson comes out with a black hole vacuum, I am ditching my old one.
Of course, if your parents did go to a parallel universe, the people returning might not be your real parents at all, but some alternate version of them. In fact, you might be living in an alternate universe right now. Look around at your stuff. Is it really your stuff? How can you prove it is not exact duplicates of your stuff, altered slightly, but not enough to see with the naked eye? This could be precisely why we keep losing socks and car keys.