Cosmology has got to it's current state through decades of work by many people of great talent. And although scientists are making progress there is still a lot that nobody knows at present.
Please remember that this work deals with pysical reality at levels greatly removed from out personal experience where things often behave in ways that are quite thoroughly at odds with our intuitions.
Alan Guth's _THe Inflationary Universe_ is just one of the books that tries to bring the discoveries of cosmologists to a lay audience
I am not sure what you mean by "100% atheist" but if you mean absolute certainty that there is no God or gods of any sort then I don't know anyone who I could definitely say fits that description. But the simple fact is that proposing a God is not a good answer to explain the universe so it is not a significant issue to me.
And I am really not sure why our lack of knowledge should not be a reason to be "100% agnostic" - isn't the whole point of agnosticism NOT to come to an absolute conclusion without sufficient knowledge ?