White Hole
Overview
A white hole is the theoretical opposite of a black hole. Some theories postulate that some, though not all, black holes might have a white hole as a counterpart.
In spacetime there is the concept of a light cone. If black holes possibly had other regions that an observer might be able to access in their future light cone, then a white hole becomes a possibility as an endpoint besides a singularity. Going further, one might even be able to access alternate universes.