A [universe] wears [its] bruises like a badge of pride

Apparently, our universe hasn't got the gumption to leave.

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.