Man has, since primitive times, tried to answer all questions about life, including questions beyond his understanding. This makes the concept of
god a primitive concept, by definition.
The fact that different primitive cultures guessed at the answers differently means someone guessed wrong.
Atheists don't deny that there are tremendous unknown forces in the universe. They just don't like to have someone with no more knowledge than themselves give the forces a name and ask for donations. If someone defines god or spirits in general terms, rather than "looks like a man" or "floats like a sheet", atheists are on board.
Atheists don't deny the existence of these voids in human understanding, or that there may be answers. They just don't accept someone else's answers. It's too important a topic to stop thinking about, even if the answers aren't out there.
When atheists say "There is no God" they are referring only to a God that someone else (eg: Christians) has chosen to narrowly define.
They are not referring to the existence of forces greater than themselves.
It is a favorite logical fallacy of proponents for THEIR version of God to say that HE must exist because it's so obvious that there are greater forces than man in the universe. This is a giant leap, for which there is no logical bridge.