Argument from ignorance, also known as argumentum ad ignorantiam or "appeal to ignorance", is a fallacy in informal logic. It asserts that a proposition is true because it has not been proven false (or vice versa). This represents a type of false dichotomy in that it excludes a third option, which is that there is insufficient investigation and therefore insufficient information to satisfactorily prove the proposition to be either true or false.
http://en.wikipedia.org/wiki/Argument_from_ignorance