The "Big Bang Theory" is the prevailing cosmological theory decribing the origin and evolution of our universe.
The Big Bang theory states that around 13.7 billion years ago the universe was condensed into a dot smaller than a period. At that time there was no physical matter in the universe which actually only consisted of space and time. The name "Big Bang" is a misnomer, since the universe simply expanded and didn't literally explode. As the expansion continued the universe eventually cooled down enough for particles of matter to "freeze out" of energy and collide to form heavier elements. Over billions of years these particles combined to form galaxies and planets.
The history of the universe can be described in some detail back to a time approximately 10-43 seconds after the big bang. Before that, the fundamental physics of the universe would have been fundamentally different due to the density of the matter. What occurred earlier than 10-43 seconds is currently unknown to science.
Creationists often object to the Big Bang theory on the grounds that it removes God's hand in creation. A common Creationist argument against it is the question "What caused the Big Bang?" and the closely related question "What happened before the Big Bang?"
There is a common misconception that the Big Bang means that the universe "came from nothing." Creationists use this as a launching point to claim that without introducing God, the first law of thermodynamics would be violated.
The Big Bang theory does not say that the dot came "from nothing"; it simply identifies the existence of the singularity and then proceeds from there. The answer to the question of what came before the Big Bang is simply "No one knows yet."
This lack of knowledge cannot be used to assert the existence of a God, however, since one can always ask "Who created God?" Since Creationists typically believe that every effect needs a cause they assume that the universe needed one too. See the Cosmological argument.