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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.