You can't really prove something doesn't exist.

Prove

1 Demonstrate the truth or existence of (something) by evidence or argument: "the concept is difficult to prove".

2 Demonstrate by evidence or argument (someone or something) to be: "innocent until proven guilty"

You can prove something does exist, with evidence, but just because you have a lack of evidence doesnt mean that it doesnt exist, you just have failed to prove its existence.