Our universe is expanding, and about 14 billion years ago, everything we currently see was squeezed into a very hot, very dense, rapidly expanding state. If we extrapolate all the way back, our best current theory (Einstein’s general relativity) predicts a “singularity” of infinite density, labeled “the Big Bang.” The right way to think about that is not that there really was such a singularity but that the theory itself breaks down. Someday we’ll have a better understanding of gravity and the expanding universe, which will hopefully reveal what happened at the moment we call the Big Bang.
