Here's a partial discussion:

"At the instant of the Big Bang, the universe was infinitely dense and unimaginably hot. Cosmologists believe that all forms of matter and energy, as well as space and time itself, were formed at this instant. Since "before" is a temporal concept, one cannot ask what came before the Big Bang and therefore "caused" it, at least not within the context of any known physics. (At least one cosmological theory, however, predicts that our universe's Big Bang is part of a chain reaction in which the demise of one universe spawns the birth of many, parallel, universes. According to this scenario, our universe may simply be part of a huge, infinitely growing fractal.)

Science tells us nothing about the way space, time and matter behaved in our universe's earliest instant, from the time of the Big Bang to 10^-43 seconds later. Space was certainly expanding--violently--and from this expansion of space was formed a highly energetic soup of particles and antiparticles.

The energy was so great during the this so-called Grand Unification Epoch--a fine-sounding name for the period from 10^-43 to 10^-35 seconds after the Big Bang--that all matter and energy was essentially interchangeable and in equilibrium. What's more, electromagnetism and the two nuclear forces were as one (gravity, the fourth and weakest force, had separated from the other three at the beginning of the Grand Unification Epoch)."

From http://archive.ncsa.uiuc.edu/Cyberia/Cosmos/InTheBeginning.html