Argument from ignorance
A proposition is true because it has not yet been proven false.
Also known as appeal to ignorance, it represents a false dichotomy in that it excludes the possibility that there may have been an insufficient investigation to prove that the proposition is either true or false.
False positives such as the placebo effect, superstitions, correlation-causation fallacies, absence of evidence, negative results and experiments with small sample size:
“There is no evidence of aliens, therefore aliens don’t exist.” Using an antibiotic one day and stopping because the desired effects haven’t kicked in (the drug would have worked had it been used for 7 days).