The universe: you thought it ended with a wall that just went on forever, right? Me too. But apparently that's not the case. Apparently it's like a bubble with-- "wait, like a bubble in my bong?" *rolling eyes* Sure, it's like a bubble in your bong. But, instead of being surrounded by a bunch of dirty water you never change, it's surrounded by other universes. Other universes that, occasionally, run into ours. Neenee noonoo neenee noonoo neenee noonoo! Get it? That was supposed to sound like the beginning of 'Twilight Zone'. "Yeah? Well it sounds like shit." WELL YOU SOUND LIKE AN @$$HOLE.
The multiverse theory (or one of them, anyway) suggests that just outside of our universe, other universes are appearing and disappearing, each in their own bubble of space-time. Generally, these universes don't get close enough to interact, but sometimes a universe will appear right next to ours, and when that happens, we get smacked.
Getting smacked by an entire universe would definitely leave a mark. Specifically, it would leave a disk-like pattern in our universe's cosmic microwave background radiation, and this is what a group of cosmologists from University College London have been looking for. Somewhat incredibly, they managed to find some of these bruises. Four of them. And it's ten times more likely that the four marks are universe collision signatures than that they are anything else that we know of.
Oh man, most of that was like, light-years over my head. But I do understand getting smacked, and I'm more than a little upset our universe isn't the one doing the hitting. "Noooooooo, our universe is a kind and gentle universe and would never do something like that." Ha, is that what you think? Let's ask Stephen Hawking. Stephen, what do you think about all this? "ZIPZAP RIBBLE RABBLE." See? See what our universe has done to him?! I'll be at the bar.
Thanks to dozer, who claims he doesn't believe in other universes. Whoa -- you do know every time you say that one falls down dead, right? "Yes." Cool let's chant it.