The Big Bang Theory attempts to explain how the universe was created around 13.8 billion years ago from an infinitely dense and hot singularity. It holds that all matter in the observable universe was compressed into a tiny point that began rapidly expanding and cooling. Two key scientists who developed it were Hubble and Lemaitre. Black holes are theorized to be the power behind the Big Bang, as their intense gravity can compress matter into a single point, and could trigger another "Big Bang" by consuming all matter into a single black hole that causes re-expansion. The theory remains controversial on questions like what caused or predated the initial expansion.