What do atheists believe happened before the “big bang”?
Why are you asking this question of atheists? We have no way of knowing what happened before the Big Bang. Contrary to what Bible believers say about the Big Bang, it does not necessarily mean that there was nothing before. There are several theories, but we have no way of knowing for sure. All we can say is that the occurrence of the Big Bang is a logical conclusion derived from the observed expansion of the universe, and several predictions derived from Big Bang theory have been verified, so that it is reasonable to accept that it occurred. Added: @B, you should look it up yourself instead of believing what you see on lying creationist web sites. Darwin never recanted on his death bed.
