White hole / black hole theory
It's a mystery. Modern physics cannot accurately describe what goes on at the singularity where both huge gravitational effects and quantum-scale events become significant. (Relativity and Quantum Mechanics have yet to be linked....e.g., through some theory of Quantum Gravity)
The mass/energy may simply stay there or it may explode into a new universe. Who knows?
A white hole is another mathematical solution to (I think) General Relativity...however, there is no evidence for their existence. It is the opposite of a black hole (point at which nothing can enter?). There is some speculation (mostly in Sci Fi it seems) that there is a wormhole/tunnel between black holes and white holes. But there's no evidence that wormholes exist either.