Why "I don't know" is often the best answer: The Appeal to Ignorance

The appeal to ignorance fallacy occurs when individuals take the lack of information about a certain subject as proof of either its existence or nonexistence. Essentially, it’s the belief that something is true because we don’t know it isn’t true, or, conversely, the disbelief in something because we don’t know that it is true. It is often used to justify a position that lacks a certain amount of evidence: ESP, UFOs, etc. When we don’t have an adequate explanation, it is rational to say that we just don’t know – not to jump to a conclusion one way or another. Our knowledge about the world continues to grow, and just because we currently lack an explanation doesn’t mean that there is none or that the explanation is supernatural, paranormal, or otherwise “unnatural.” This fallacy is committed if, when someone “pleads the fifth” or has no alibi for a crime, we assume they are guilty. (Other names include argument from silence, ad ignoratiam, and appeal to ignorance).

How this applies: Intelligent design is based on this fallacy: the argument is that “gaps” in scientific knowledge are proof of God and anything currently unexplained must be divine. Structures that have been proposed as “irreducibly complex” have, in fact, been rejected and usually provide good opportunities for scientists to earn Nobel Prizes and other accolades in discovering how they evolved. Additionally, anytime someone dismisses a scientific consensus on something because "we weren't there to see it" or because there are "gaps" in the fossil record, they are committing this fallacy. This fallacy is also committed when someone uses lack of scientific evidence about God to disprove God or when they say that because science can't disprove God that therefore God must exist.

Examples:

• “The vet can't find any reasonable explanation for why my dog died. See! See! That proves that my neighbor poisoned him! There’s no other logical explanation!”
• “We haven’t proved that Big Foot doesn’t exist; that means that he does.”
• Joe McCarthy said he was presenting to the Senate cases in which it was clear that individuals had Communist connections. With one case, however, he said "I do not have much information on this except the general statement of the agency…that there is nothing in the files to disprove his Communist connections." His argument was that because there was no evidence against a Communist connection for a person, that person must be working with the Communists. Source: Richard H. Rovere, Senator Joe McCarthy (Methuen, 1960), pp. 106-107.
• There is no evidence for the Loch Ness monster; that is proof that the Loch Ness monster does not exist.
• Since you cannot prove that ghosts do not exist, they must exist.
• Since no one has been able to prove Iraq doesn’t have weapons of mass destruction, we can conclude that Iraq must have them.
• Obama hasn't proved he was born in the United States. Therefore, he wasn't born in the United States. Furthermore, no evidence he or anyone else brings forth can prove he was born in the United States. (The latter claim might also be branded the appeal to the impossible demand. The argument both demands proof and announces that nothing will count as proof.)