The Big Bang Theory is the most widely accepted hypothesis for the origin of the universe as we know it. It's implications are somewhat vague, as there is no concrete way to determine what really happened to create our universe, but it still holds water.
The basics of the Big Bang Theory is that a singularity was formed, a very small ball with infinite density and unimaginable heat. This singularity's expansion is what created the universe, essentially, a big bang.
