You don't need to examine every bit of universe to prove that something is true or not. You have to have a system, a tool that is proven to be correct and use it. 1000 x 1000 = 1 000 000 : how do I know this? Because I counted every unit of the first thousand and summed it thousand times with every sum of other thousands? No, I used math. I can say that monster doesn't exist if I have some apparatus to prove it. But in this concrete case the apparatus is non-existent and since the vague definition of monster it is also meaningless.
And by the way, I did have an opportunity to speak with professor who is dealing with a string theory, and it is not just this theory contradicts other theories, it has 5 different interpretations and only two of them are somehow connected, otherwise they still kind of contradict themselves (or at least do not entirely support each other). And he also said that many people who work with string theory are becoming slowly aware, that it is not entirely the right way. That only implies that we do not understand it entirely yet, but it is far too soon to claim that nothing can be proven. If this is the case, why so many people try?