Imagine this for a moment: A black hole is a microscopic mass so dense that everything coming within a certin proximity to it (the event horizon) is sucked in and mashed into that tiny point likely being destroyed, reduced to its most basic elements by the violent and extreme force of the black holes gravity. Imagine that at one point after perhaps billions of years that point reaches some as yet not understood threshold and bursts out in a wild expansion of all that stored up energy, but the expansion is so violent that it somehow crosses a border that we can't yet detect into an alternate dimension of reality. There you have the big bang of an alternate universe.
This is a simplistic (and admittedly uninformed) summary of a theory that some physicists have put foreward. Very theoretical but nonetheless interesting.