Atheists & Agnostics

That's one that we really don't know for sure. The big bang theory is not universally accepted, and some recent observations cast a bit of doubt on it.

One has to be very careful with the word "know" in science. Do we know that the universe is expanding? No, we don't. We observe that more distant objects are more red-shifted than closer objects. We assume that velocity is the only thing that causes red shift, and from that we deduce that distant objects are moving faster, and that therefore the universe is expanding. The weak link is the assumption that velocity is the only thing that causes red shift.

We "know" that the universe is expanding with the same degree of certainty that the 19th century geologists "knew" that continents don't drift around. But there is no final point where we can say that we know all there is to know, and tomorrow's discovery may turn today's "knowledge" on its head.

As for "god", no we can never know that there is no god. We cannot know that there are no unicorns. Perhaps on some distant planet the process of evolution resulted in what we might call a "unicorn". Until we have examined every square foot of every plant in the entire universe, we cannot "know" that there are no unicorns.