String Theory is relatively new Quantum Mechanics, in its infancy and not fully accepted, but its following among scientists is big today and growing bigger all the time. You can watch a three hour documentary on String Theory. The last hour of it deal extensively with the Big Bang.
http://www.pbs.org/wgbh/nova/elegant/program.htmlEssentially, the String Theory says that the Big Bang has happened many times before, and will happen many times in the future. What causes it to occur? When one 'universe' collides with another. So far, there are eleven dimensions that exist in the universe - according to the mathematical model of string theorists.
Strings are smaller than an atom. Quarks are suspected to be made up of strings. If an atom was the size of our solar system, a string would be the size of an average tree. So millions of these 'strings of energy' will exist inside an atom.
String theory thinks that gravity is made up of special looped strings, called Gravitons. These particles constantly escape our universe and travel (or leak) to the other universes. This, they say, explains why Gravity appears to be much weaker than it actually is (for instance, your arm can pick up a ball, giving the appearance that your arm is stronger than the force of gravity).
