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.