I'm agnostic, but explain to me why Atheists are so certain that God does not exist. This seems ironic.

I agree that science can explain most of the inner workings of the universe and that it dispels most of the claims made by major religions. However, none of us still knows with absolute certainty whether or not God exists. Is Atheism based in a faith that proof of God's non-existence will someday be uncovered? That sounds ironically similar to religion.