Not everything is yes or no. Do you know if aliens exist? It's a possibility, but we've never seen nor made contact with them. Denying the existence of something just because you haven't seen or heard from it is rather silly. I'm not saying there is a God but I'm also not saying there isn't. We don't know.
If you don't believe in any god due to lack of evidence or whatever, but still keep in mind the possibility of one existing, that's called Agnostic Atheism. (I fit into this category)
If you feel certain about no god existing, that's Gnostic Atheism.
Then there's Agnostic Theism where you believe in a god but keep in mind the possibility of one not existing, and Gnostic Theism where you feel absolutely certain about a god existing.