Atheism is religion.
Anyone who thinks they have a clue about the origin of existence is arrogant and presumptuous.
There is no answer to the fundamental question:
Did something come from nothing?
Or did something always and eternally exist?
There is no answer that is comprehensible to the human mind.
You can call that God or make up any sound or concept, it is still beyond human comprehension.
And if you don't know the first thing about anything so fundamental as (or to) existence itself, then you really don't know anything for sure because everything else is ultimately relative to the fundamental.

I really don't understand why people equate atheism and science.
While the two don't necessarily go hand-in-hand, generally speaking the more scientifically inclined one is the less need they have for religion. That said there have been some prominant scientists who are/were religious. Among them George Lemaitre (Catholic priest and originator of the "Big Bang" theory) and Francis Collins (who headed the Human Genome Project).