I used to define myself as "agnostic", as in "well I don't know, right now I don't have any reason to believe in <choose your God here>, but if you could prove it I would obviously change my mind".
But then I changed my mind: now I believe that agnosticism is just a "shy" way of being atheist, somehow trying to say "I don't believe in a particular deity, but I can't commit to saying that I believe there is no deity". But that's the whole point of a belief: I could be wrong, and it would be okay to change my mind. A belief is not a proven fact.
So I am an atheist: I don't believe in any particular deity, and in fact I do believe that there is no such thing. But obviously if you proved my belief wrong, then I would change my belief :-).
Like if the gases of a nebula got rearranged to spell out “God is real”, then sure yeah I guess they are real.