Dismissing the evidence somebody brings as a conspiracy, or marginalizing it because it is incomprehensible to the listener, is an argument from incredulity. Even though something may seem fantastical to the listener's current ability to perceive it, it can still be true -- it is up to the evidence, as well as both inductive and abductive arguments made from the evidence, to determine the validity of the claims.
"The Argument from Incredulity: The popular fallacy of doubting or rejecting a novel claim or argument out of hand simply because it appears superficially "incredible," "insane" or "crazy," or because it goes against one's own personal beliefs, prior experience or ideology. This cynical fallacy falsely elevates the saying popularized by Carl Sagan, that "Extraordinary claims require extraordinary proof," to an absolute law of logic. See also Hoyle's Fallacy. The common, popular-level form of this fallacy is dismissing surprising, extraordinary or unfamiliar arguments and evidence with a wave of the hand, a shake of the head, and a mutter of "that's crazy!"" ~Master List of Logical Fallacies
