appeal to ignorance

Concluding something is true because it hasn't been disproved, or false because it hasn't been proved.

An appeal to ignorance treats a lack of evidence against a claim as evidence for it (or a lack of evidence for it as evidence against it). It is not fallacious when a thorough search was made that would very likely have found the evidence if it existed.

Examples

"No one has shown that this charm doesn't bring luck, so it does."
Not fallacious: "We checked every pocket and the car, and found no keys, so the keys aren't here."