The Big Bang Theory is the leading explanation for how the universe began. At its simplest, it talks about the universe as we know. It starts with a small singularity, then inflating over the next 13.8 billion years to the cosmos that we know today.
What is a "singularity" and where does it come from? Well, to be honest, we don't know for sure. Singularities are zones which defy our current understanding of physics.
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According to the big bang theory, the universe began by expanding from an infinitesimal volume with extremely high density and temperature. The universe was initially significantly smaller than even a pore on your skin. With the big bang, the fabric of space itself began expanding like the surface of an inflating balloon – matter simply rode along the stretching space like dust on the balloon's surface. The big bang is not like an explosion of matter in otherwise empty space; rather, space itself began with the big bang and carried matter with it as it expanded. Physicists think that even time began with the big bang. Today, just about every scientist believes in the big bang model.