The argument from ignorance, also known as appeal  

to ignorance or argumentum ad ignorantiam,  is a fallacy that occurs when something is  

claimed to be true simply because it has  not been proven false, or vice versa.

This fallacy asserts that a lack of evidence  against a position is evidence for it,  

or that an argument is necessarily false  because it has not been proven true.

Everyday Example:

An individual claims that there must be  life on other planets because no one has  

been able to prove definitively that there isn't.

In this instance, the person is basing  their belief on the absence of evidence  

to the contrary rather than on positive evidence  supporting the existence of extraterrestrial life.

This argument assumes that the lack of disproof is  equivalent to proof, which is a logical fallacy.