Grog said:You can't prove that something doesn't exist. That's one of the most basic tenets of logic.
This is an often quoted and completely inaccurate statement. I see this all of the time on the internet (though I can't prove it.)
You *can* in fact prove things don't exist, especially in a logical system where an element's existence would contradict a basic axiom of the system. For example, to use a famous problem - a construction of a square with the same area as a circle using only a straight-edge and compass does not exist. The technique has been proven to not exist.
However, saying that you haven't found something isn't proof that it doesn't exist, if that's what you mean.