A white hole is the opposite of a black hole. It's like a cosmic gusher, with matter pouring out of it. If matter goes through a black hole tunnel, it comes out of a white hole at the other end.
Some calculations show that a white hole that is not rotating could get turned inside out by gravity, having a black hole form around it. But if the white hole is rotating, it can't suffer this fate, and will not be hidden from the rest of the universe.