Atheists often make the claim that the burden of proof lies with the believer, not the unbeliever. They would ask whether you can prove that the nonexistence of anything. Rather, it should be up to the person who makes the positive claim to provide proof, not the people trying to prove a negative.
However, there are many things that are invisible that I might have trouble proving. Let's take electrons, for example. I've never seen one myself. Many people believe in them simply on the authority of scientists. People also believe in God, even though they can't see God, because they take his existence on the basis of authorities. What's the difference?