As atheists, how do you respond to this statement: "You believe something came from nothing."
"No, I believe that everything occurred according to the natural progression of events. Physics, chemistry, and biology have all provided fairly reputable and verified theories regarding those laws. If pressed about the origin of the Universe, and where everything came from, my answer is 'the last Universe, which collapsed into a cluster of singularities and then exploded violently outward as a result of too many singularities trying to occupy the same point in space. Neither energy nor matter are ever truly destroyed, they just transfer into other states, eventually there's a collapse and reshuffle."
The fact that the last bit regarding what the Bang spread, how, and why, is largely my own hypothesis and is only so relevant, but it's still a more plausible answer than 'an invisible man in the sky made something where there was nothing.' So really, it's the religious who believe in something from nothing, at the whim of some cosmic sentience no one can reputably contact.
Big Bang? I don't know this theory well enough to conjecture.
Apologists make this comment to suggest non-theists' beliefs suffer the same difficulties to explain, a middle road fallacy. It's also the equivocation fallacy because although assuming all non-theists believe in the big bang then the universe having a beginning implies it came out of 'nothing' but most theories predict alternate states instead. But still all highly hypothetical.
The only honest answer is we don't know. We don't know, is a valid answer. I then point out to theists they don't know either. They claim they know, but they can't and don't support their claim with anything tangible. Inserting, therefore, God did it every time science says we don't know, is something theists have done since the onset of science. Every time they do that, science demonstrates they're incorrect, and then they just find a new thing to say therefore God did it, about. Unfortunately, until we become Gods ourselves, and know every knowable thing, we'll be dealing with the God of the gaps argument.
Paul hit it on the head. This is the closest we can get to explaining something out of nothing.