"A star becomes a black hole when it collapses and reaches such a high density that its gravitational field exceeds the escape velocity of even light photons. Astronomers theorize that a change in time/space occurs. This creates many probabilities, including (1) entrance and exit points to other dimensional realities, (2) birthplaces of future stars, (3) an ability to harness the powers of time travel, and (4) an entrance into an antimatter universe." (Lyssa Royal & Keith Priest, The Prism of Lyra)
Some people theorize that if black holes exist, so must white holes.
See also: White Hole for more information.