The Big Bang theory explains where the universe came from.
Have you ever heard (or said yourself) something like this: Out of nothing, there was an astoundingly large explosion and – ta-da! -- the universe was created.
Unfortunately, that's not actually what the big bang describes. Rather, it tries to explain the expansion of the universe. It doesn't say how the universe came to be but what the universe did to become gigantic.
Although most of us envisioned the big bang as an explosion at least worthy of a Jerry Bruckheimer movie, we were wrong about that as well. Sure, the not-even-microscopic center of everything inflated to an easily discernible size in less than a trillionth of a trillionth of a trillionth of a second -- but no fireballs or clouds of thick smoke were engulfing muscled heroes who might've been around to dive out of the way [source: Khan].