Theists are accused of committing a “God of the gaps” fallacy, which is to assume (insert) God (into the gaps in our understanding). Atheists say that this assumption (faith) is irrational and wholly unsupportable by science or reason. They say they would rather rely on science and reason, and conclude there is no scientific or rational support of the proposition that God exists.
Atheists tend to be materialists, meaning that they believe that the “world” (all that exists) is material, only. They tend not to accept that anything other than the material world exists.
An exploration of these contentions will show that the material world is not all that exists and that atheists who subscribe to the modern view of the multiverse, which is used to explain problems for which science has no answers if only one universe exists, is a resort to the same kind of fallacious thinking pinned on theists. The God of the gaps argument against faith and belief in God conflates science and reason, and a materialist worldview conflates scientific knowledge with all possible knowledge of the world.