Similar to the Argument from the Negative Fallacy, this fallacy occurs when the arguer, because he/she can't (or won't) produce the necessary evidence to support his/her claim, attempts to unfairly shift the burden of proof onto the respondent. "Well... you can't prove the opposite of my stance; therefore, my stance must be correct."
Example: Obviously, 'squatch exist because nobody can prove they don't.
Where does the burden of proof lie? Is it up to you go turn over ever inch of earth in the world to "disprove" this?
Example: Did you know that there are puppy dogs riding unicorns backward beneath the surface of Pluto?
Just because you can't prove this wrong doesn't mean this argument is acceptable as truth.