"Agnosticism" is the view that God's existence is unknown or unknowable. However, too many self-proclaimed agnostics are mislabeling themselves for lack of a clear definition of what knowledge means here. While it can't be proven that God doesn't exist, it can be known. Let me explain.
It can't be proven that God doesn't exist because God is a malleable concept. For example, at one time some people believed that God had created all life on Earth exactly as we known it today, including humans and all other animal species. Some of those same people have come to recognize the fact of evolution, which disproves their previously-held belief. Now, instead, they believe that God engineered evolution, or some other conveniently not-yet-disproved story.
No matter how much evidence we uncover that the universe functions without the help of a divine force, we'll never be 100% sure that there isn't a God. Maybe God started the Big Bang and hasn't intervened since. Maybe he made it look like he doesn't exist by planting fake evidence. However supremely unlikely that may be, there's still a teeny-tiny chance it's what happened. Likewise, maybe the evidence backing evolution was planted. Perhaps by God, perhaps by the Illuminati?
There's a non-zero chance that unicorns, invisible to the human eye, are roaming our streets and skies as I write this. And yet, I don't say I'm agnostic towards invisible unicorns. Same goes with evolution, gravity, and, of course, God.
As it turns out, technically, we can't know anything with 100% confidence. And that's OK, because the bar for knowledge and facts is lower than that. Not much lower, mind you. But if you're 99% sure God doesn't exist, do me a favor, call yourself an atheist.