The Big Bang possibly was the collision of other dimentions known as membranes. One membrane colliding into another resulting in a manifestation of the physical universe. Time is a function of Space and vice versa. Time does not exist prior to T=0, so postulating a "before" is moot. The Big Bang is humanities attempt to trace back the events that occured which perpeturated the stretching of space/time and the formation of the elements/gas/stars/sentient life.
The events postulated are continually being confirmed by observations made by the hubble telescope and the disciplines of science. Although it is difficult to imagine the universe being a singularity
it is likewise difficult to imagine what your own eye looks like without the aid of a mirror. Humans are reduced to perceiving reality through the physical laws of nature. If something is outside of humans ability to percieve it then how would we ever know it exist? This seems to be the main argument I get from many Theist. And although one can always say "but how do YOU know" I contend that Occams razor is reason enough not to introduce duality into our current state of understanding. Simply saying I dont know will suffice for me.