This says it better than I can:
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What Can Science Prove or Disprove?
To understand why "God does not exist" can be a legitimate scientific statement, it's important to understand what the statement means in the context of science. When a scientist says "God does not exist," they mean something similar to when they say "aether does not exist," "psychic powers do not exist," or "life does not exist on the moon."
All such statements are casual short-hand for a more elaborate and technical statement: "this alleged entity has no place in any scientific equations, plays no role in any scientific explanations, cannot be used to predict any events, does not describe any thing or force that has yet been detected, and there are no models of the universe in which its presence is either required, productive, or useful."
What should be most obvious about the more technically accurate statement is that it isn't absolute. It does not deny for all time any possible existence of the entity or force in question; instead, it's a provisional statement denying the existence of any relevance or reality to the entity or force based on what we currently know. Religious theists may be quick to seize upon this and insist that it demonstrates that science cannot "prove" that God does not exist, but that requires far too strict of a standard for what it means to "prove" something scientifically."