We don't know anything about the Big Bang for sure, and it's possible we never will, as beyond a certain point, there was no light, so there is a limit to how far back in time telescopes can see. This occurs at about 380,000 years post-big bang and is known as the epoch of recombination. Before this, the universe was too hot for electrons and protons to pair up and form hydrogen and unbound electrons scatter light so until there was hydrogen, the universe was opaque.
However the furthest back we have seen is maybe 700 million years after the Big Bang. So there is still plenty of time, literally. The James Webb telescope, due 2018, will be capable of extending our view back to 100 million years ABB. What it will see there is debatable, as whilst with difficulty we can discern young galaxies at these distances, if there were yrt no galaxies (current thinking is that unassociated but giant stars predated the formation of galaxies) these may be way too faint even for James Webb.
Standard cosmology applies from the present time back to about 1/100 second after the Big Bang. Before then, particle physics and quantum cosmology describe the universe.
There are scientific models where the Big Bang and thus the entire universe arose from simply a random quantum vacuum fluctuation in a particle field, and it is this process that Hawking is referring to in my above link re the possibility of the God particle destroying the Universe, a BB in reverse, if you like.
The evidence for the Big bang as the beginning of what we know as time and space is therefore pretty overwhelming, but as for what happened to "cause" BB, or what existed "before", is really outside the realm of science, not because scientists give up, (most believe that "something" existed before BB) but because we can never know what happened "before" BB, as by definition there can never be anything to see or test or measure before anything existed.
Personally I like the theory that there is a multiverse, think of it as each universe pictured as a membrane (or "brane") floating in the multiverse, and if one happens to touch another, this causes a BB. But there will (probably!) never be a scientific test of this as we by definition can't see outside "our" universe - as for us there can by definition be nothing else that exists outisde our universe, otherwise it wouldn't be a "universe". This of course doesn't EXCLUDE other universes but we can't reach them, as our own universe is infinite, and so doesn't have a border to cross to go outside it.
I like the notion that just maybe [url=http://www.dailygalaxy.com/my_weblog/2013/06/the-great-attractor-is-something-is-pulling-our-region-of-the-universe-towards-a-colossal-unseen-mas.htmlThe Great Attractor[/url, a highly mysterious region in the Universe which is pulling our Milky Way - and tens of thousands of other galaxies - toward itself at 14 million miles per hour, is the spot where the event took place, but it's all a bit mad! I mean, the very notion (100% factual btw) that, apart from all other motions, I am heading that way at 14 million mph is more than enough to do my head in.
