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.
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?" These are simply answered by "No one knows yet." The counter-question could be asked "What created God?" Since Creationists typically believe that every effect needs a cause they assume that the universe needed one too. See the Cosmological argument.