but just to throw a spanner in the works, M-theory, or more especially brane-world cosmology speculates that the 'big bang' as such never happened.....it speculates, as QS alluded to, that our universe is a 3 dimensional brane floating in a higher dimensional space along with a huge number of other 3-D branes which, are separated by a tiny amount in the higher dimensional space....Negative energy between 2 of these branes attract the branes as they ripple in the higher dimensional space eventually touching in one spot, resulting in a huge energy release as they touch and rebound....This energy release fills the 2 branes and in our particular brane world results in all the matter we perceive today....The branes are repulsed further and further apart over time though the higher dimensional space (or Bulk for short) a phenomena we perceive as 'Dark Energy'...After say a trillion years, the repulsive force dissipates and the 2 branes are started to be mutually attracted by gravity until negative energy takes over and they touch again repeating the cycle......i.e no big bang, rather an eternal 'Big Splat' cycle
confused?......I was for a long time before getting my head around brane-world cosmology....still it's one of many theories, but for me it's a simple theory, that makes sense, answers the key questions that other theories do not, it helps marry General Relativity & Quantum Theory, and lastly is backed up to a degree by data from the WMAP satellite currently in orbit