Literally “Against Ignorance”, this fallacy assumes the truth of a statement for no direct evidence against it. For example, the Flying Spaghetti Monster is real, because science can’t disprove it.
A better stance to start from is a version of the null hypothesis. That is, if there is no evidence for it, then assume it doesn’t exist until you find some evidence that maybe it does. Please note that evidence means more than “look – it moved”, but rather a repeatable experiment with conclusive data that is tested by multiple people under proper conditions.
If you feel that your statement might borderline on this logical fallacy, insert Flying Spaghetti Monster or Rainbow Pooping Unicorn and see if the same logic you used proves that Flying Spaghetti Monster’s or Rainbow Pooping Unicorn’s now exist.