Let's start with Wikipedia.

http://en.wikipedia.org/wiki/Big_Bang

Read the page and the caption underneath the model diagram.

http://en.wikipedia.org/wiki/Cosmogony

From Overview:

The Big Bang theory is the prevailing cosmological model of the early development of the universe. The most commonly held view is that the universe was once a gravitational singularity, which expanded extremely rapidly from its hot and dense state. However, while this expansion is well-modeled by the Big Bang theory, the origins of the singularity remains one of the unsolved problems in physics.