argumentum ad ignorantiam (i.e., argument to ignorance). This is the argument that says something must be true because it has never been proven false.
On its face, this sounds like it might be a fairly reasonable tactic. Why is that? Because it’s really a question of who holds the burden of proof. In the case of argumentum ad ignorantiam, the burden of proof should fall to the person employing the tactic.
For example, one might say that Sasquatch is real, because no one has proven that Sasquatch does not exist. Of course, given the lack of evidence for Sasquatch, the burden of proof falls on the one making the claim of Sasquatch’s existence, to prove it by evidence. You can see the appeal of this argument when claiming the existence of the supernatural, because clearly it can’t be proven; therefore the burden of proof is fallaciously shifted to the skeptic by suggesting that the phenomenon has never been proven not to exist; therefore it must.