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.
White holes, which are basically reversed black holes, may not even exist in our universe, but you can create a perfect substitute for them in your kitchen. Water flowing from a tap obeys exactly the same laws as white holes.
White holes are basically black holes in reverse: They eject all the matter sucked into a black hole. We've never observed one, and most astrophysicists suspect they don't exist - all because they're not the exact opposite of black holes.