How can anyone just say, with 100% certainty, that there is no higher power?

I always argue with my friend (Atheist) about this but I just don't get how anyone could be completely positive on something so confusing.

I do not believe in any organize religion or any supreme being for that matter. I just don't get how anyone human being with such little knowledge of the universe can say that there is noting more than this life or there is no deity like being. When I think about how big our universe is, the huge amount of things going on that we don't understand, the huge amount of things we don't know or will never see, it doesn't make sense to me that anyone could make an intelligent decision on such a question. The way I see it, it is like if there was a billion page book and you saw someone read the first sentence of the book, close it and then try to tell you what it was about. I am not saying there is a god. I am not saying if there was a supreme being it would have any indifference to my existence or anything like that. I am just saying that I don't know shit and for me to attempt to make any sort of judgement on this matter would be foolish to say the least.
Some people say I am agnostic, I would rather say I am not comfortable answering that question due to my lack of knowledge on the subject matter.