First of all, what is a black hole? It's is what is left when a massive star collapses, after it has used up all its fuel. The star collapses under the force of its own gravity, eventually reaching a point where the gravity it generates is so strong, that not even light can escape. The core of a black hole is so dense that it sinks through the fabric of spacetime, creating a hole. That core is known as a singularity. The "opening" of a black hole is called the event horizon. It is the point of no return for any object that gets caught in the black hole's powerful gravitational field. Once something passes the event horizon, it is gone for good.
However, black holes aren't as black as we originally thought. They can and do emit tiny particles of radiation, and this emission can cause a black hole to lose mass. As its mass decreases, its temperature rises, and particles get thrown our faster, so the black hole gradually loses mass more quickly. Eventually a black hole's mass becomes extremely small. What happens then?
One theory is that the black hole disappears completely in one big final burst of emission, equivalent to the detonation of millions of hydrogen bombs. Or the black hole may wink out of existence. What happens then to all those objects that fell into it? Stephen Hawking surmises that these objects go off into a little baby universe all their own: a small, self-contained region that branches off from our own universe, eventually rejoining our region of spacetime. There could be an infinite number of baby universes out there, so small that we wouldn't even notice them branching off.
The American Astronomical Association contributed to the TV portion of this report.