14 Apr 08
Wikipedia is your friend: http://en.wikipedia.org/wiki/White_hole
I think the idea is that pretty much none of the equations of physics distinguish forward-flowing time from back-ward flowing time, so anything that can happen in one direction should be possible in the other. If there exist objects that suck in matter, like black holes, there could be objects that expel matter in exactly the opposite way: white holes. The problem is that while black holes are stable, white holes are not and so a hypothetical white hole wouldn't exist for any substantial length of time, which is why we don't see any of them and don't expect to.