White holes are the opposite of black holes, objects into which nothing can enter but are constantly spewing out matter. They were thought to be completely hypothetical, more a mathematical oddity than a real thing...but we may have seen one.
We've talked about white holes before in some detail here and here, but the basic idea behind them is that the laws of physics aren't comfortable with things that happen in only one direction. In other words, if black holes exist, then it should be possible to reverse the equations governing them so that you get something that's reversed but otherwise identical. That's what a white hole is.
Of course, just because something can happen going both forwards and backwards in time doesn't mean that, in practice, we'll actually observe both of those phenomena. (The fact that entropy only increases when the laws of physics say it could just as easily decrease is a famous example of this, but we'll leave a discussion of that for another day.) At its most basic, white holes simply wouldn't be as stable as black holes are, and it seems that they would collapse almost immediately under the weight of its own gravity.
Here's where things get interesting. A gamma ray burst back in 2006 didn't fit with our understanding of where they come from - its long duration (102 seconds) meant that it had to be created in a supernova explosion, and yet there were no supernovas there for it to have come from. Its discoverers actually said that "this is brand new territory; we have no theories to guide us."
Now, five years later, it's being suggested that we might actually have caught sight of a white hole. The fierceness and duration of the explosion could well fit with a white hole briefly popping into existence, spewing out some matter, and then quickly collapsing into itself, resulting in this massive explosion. Although it's not the most likely explanation - after all, it invokes something that many astronomers have concluded is exceedingly unlikely, verging on impossible - it can't be immediately discounted.
The trouble is that we've found out all we're going to from this particular burst, so all we can do now is wait for another of these strange hybrid bursts and see how it behaves. If these hybrid bursts really are white holes, then the universe is about to get a lot stranger.