Nobody can say with absolute certainty how the universe came into being and I know of no atheist hellbent on going around trying to tell everyone else that such is the case. Yet we surely have enough preachers hellbent on promoting the idea of the "God theory" as if it were a given.
However we do have compelling evidence that there was a big bang and scientists are still researching the subject. We also have overwhelming, irrefutable evidence that life is the result of evolution by natural selection.
I think that the old excuse, "did not witness" argument was poo pooed ages ago by sheer commonsense and logic.
Forensic detectives do not witness the murders they investigate yet are able to collect enough compelling evidence to convict a murderer..
Even without fossil and carbon dating evidence (of which there are mountains) we are still able to conclude evolution through natural selection by an overwhelming abundance of existing, live evidence.
So, anybody is entitled to reasonably assert that there is no such thing as God. Creation (the lynchpin of the existence of God) has been completely overturned and there is no evidence whatsoever of a God, Gods or any other supernatural phenomena.
Nobody is entitled to reasonably assert that there is a God and anybody who does say that there is a God is being totally unreasonable.