Good questions, all.
Does the conventional "big bang" theory even posit that matter has always existed? They estimate an age of the Universe, so they're implicitly arguing that before that, it didn't exist.
The best estimate which can be made is that put forth in the General Theory of Relativity. Time, space, matter, energy can be taken back to about 2-one-thousandths of a second. However space is another question. Without space, matter cannot exist. Without time, events cannot occur. So the Big Bang began at a moment just prior to the existence of matter, time, energy, or space. At least this is what Einstein, Eddington, Hoyle, Hawkin, Penzias, Wilson, Jastrow, and a plethora of other atheist astrophysists, despite their a prior commitment to finding a God-less universe.
So you must determine whether something came from nothing, or everything came from something. The eternal steady state theory and all of its derivitives and distinctives have been abandoned on the asheap of atheistic history.
The other question which you would not address is first life. It is the preeminent problem for the darwinist atheist.