I'm not sure I agree. Of course, very strictly speaking you are probably right. So we can never know or prove anything. We can not prove that god doesn't exist, or that the flying spaghetti monster doesn't exist, or that invisible dragons don't exist. That doesn't seem a very useful attitude, though. I think not finding evidence is some evidence, at least in a bayesian sense. And OK, we can know some things - I know there is no visible dragon sitting on my keyboard right now.