Proofs exist only in arithmetic where axioms are first stipulated and logic where propositions are first stipulated. The sciences don't have proofs, what they have is surmises. There's a lot of nonsense these days about evolution being "just a theory". Everything's "just a theory", including classical physics. In the sciences you're trying to discover things, and the notion of proof doesn't exist. Science is about finding theories of the world which are intelligible to us and which give an explanation of some aspect of reality. The more precise and extensive the match between the theory and reality the more our confidence in the theory is justified. This is why observation, experiment, prediction verification, and repeatability verification are central to science. Arithmatic, logic and computing modeling are also essential tools utilized for some science. Try Test Your Scientific Literacy! by Richard Carrier for an excellent explanation of science.
Atheism isn't a claim of proof that god does not exist. Atheism is the claim that belief in god(s) is unjustified. A fact claim is unjustified because it fails as explanation, and/or lacks credible supporting evidence, and/or has credible contrary evidence. Atheists tend to consider a dependency on a firm faith in supernatural fact claims to be an approach that is virtually certain to embrace falsehoods. Atheists also tend to consider elevating such faith to one of the highest virtues to be circular reasoning.