Black holes are essentially the wood-chipper of the universe; they suck in everything around them, including light, and destroy it. White holes, theoretically, should work in reverse. The problem is that, although we’ve seen one, they should be impossible.
Based on everything we know, a white hole is something that shouldn’t exist. It would be a black hole working in reverse, which violates the second law of thermodynamics. Black holes cause entropy, a type of chaos characterized by the whole of a system being destroyed and left in as many different conditions as possible. A white hole, then, would do the opposite, taking this mess and somehow creating a whole system from it. Again, even to think of this theoretically is problematic. However, in 2006, we’re pretty sure that’s exactly what we saw way out in deep space. There was an explosion of light that, so far, eludes any other practical definition.