The Big Bang is a cosmological (study of the universe and its formation- lots of theoretical physics) theory regarding the creation of the universe. It is almost universally accepted as true. Simply put, it states that a singularity "exploded" (but there was no true explosion just expansion). A singularity is infinitesimally small (a grain of sand or so) but it had almost infinite heat, matter, and so on. And it is believed that space (the openness with no matter or energy) came from the singularity. And it was as dense as everything in the universe (all in a little "grain of sand"). Then it expanded and chemical elements formed and over a long time the elements formed galaxies, planets, everything. And the universe is still expanding.
