Unspecified Evidence

An argument supported by the claim that evidence exists, but where such evidence is not presented.

This argument is stated as "there is a lot of proof, but I can't be bothered to show you" or "there is so much proof that if it is not obvious to you then you must be willfully ignorant of it."

In itself, withholding the evidence that would support a claim is not a fallacy. But expecting someone to accept that claim based on such unspecified evidence is an appeal to authority.