How can atheists be certain that god doesn't exist? Answer: Lack Of Evidence!
You want to prove god to me? Start by finding me a phenomenon, something which not only has no explanation but not even a hypothesis to attempt to describe how that phenomenon works. Whatever this thing is, it will have to be well outside the bounds of current understanding. I wouldn't opt for dark matter or dark energy, either, because those are under investigation and the likelihood is that their causation won't be unknown for very long. If you can find such a phenomenon, you have your first step on the road to proving that maybe, just maybe, there's a god to go along with it.
But you're not done yet. The phenomenon you choose will not only have to have no current explanation; it will have to frustrate any attempt to put a rational explanation to it. More than likely, it will have to fly in the face of all current theories in physics, chemistry, and biology at minimum, and probably multiple other major disciplines as well. This phenomenon will have to be so radical and so outside the current understanding of scientific disciplines that it would almost demand a supernatural explanation.
Ultimately, though, you'll have to find a way to drag some kind of god out and tie a cause-effect link from the one to the other. That means that the phenomenon by itself isn't enough. You still need to demonstrate the existence of the god you want to tie it to in order to complete your proof.
I'm willing to assert for the record that such an event will never happen. Why? Because, as the title of the book goes, "The Answer Is Never Magic." In the roughly 500 years that science has been examining how this reality works, it has unearthed some fascinating and occasionally disturbing operational principles. Those principles have always yielded to analysis, have proven to have some degree of consistency, predictability and repeatability. There was a MECHANISM involved which could be understood and in some cases, controlled and replicated by human researchers. Never has there been a waving of wands or an incantation or a simple BREAK between the effect observed and the stimulus which gave rise to it.
Over and over and over again, the world has yielded its secrets to scientific inquiry an the scientific method. In so doing, it has debunked a millennia-old bunch of supposedly "holy" books whose contents don't hold up very well to any kind of examination, never mind scientific scrutiny. Sure, it may be possible that the success of science might one day be broken. It's also possible that pigs may fly without benefit of Boeing, Airbus, Piper or Cessna.
Probable? 'Nother matter.
The statement God exists appears to have no stable cognitive meaning because no definitive characterization of the term God can be given and it does not refer to anything within our common experience. The attributes commonly attributed to a god—omnipotence, omniscience, omnipresence—are negatives, qualities that make no real sense. They are attributed only to the concept of god and never to anything else. Theology proclaims over an over again that God is ineffable.