Well If you look hard enough you can see "evidence" all over the internet via Pictures, Video, or word of mouth. Now the validation to these claims is something that needs to be looked at. I mean giving how most know about time paradox's and how they would fuck a hefty amount of shit up, the inventor(s) of Time Travel would make it priority #1 that the subject traveling back in time remain unseen, unheard, and never remembered. That in itself could explain why there hasn't been any "clearly undeniable proof" that it already exists in the future.