What the Fallacy?!
Argument from Ignorance
The Fallacy
Asserting that a proposition is true because it has not yet been proven false or a proposition is false because it has not yet been proven true.
Why it's wrong
This represents a type of false dichotomy in that it excludes the option that there may have been an insufficient investigation to prove that the proposition is either true or false.
Example
Since no one has ever proven that Bigfoot *doesn't* exist, the enthusiast insisted that he must be real and hiding in the local park.