The Argument to Ignorance is a fallacy of irrelevance. The claim or implication that something is true because it cannot be proven false (or false because it cannot be proven true) is a common fallacy. The burden of proof is always upon the person who makes a claim – and the more extraordinary the claim, the more extraordinary the evidence that is required to substantiate it.
The fallacy is most often presented in response to a request for the claimant to supply evidence. When your debating opponent turns your request back over to you to prove him wrong, rather than proving their case with strong evidence, he or she is committing the fallacy of The Argument to Ignorance.
The argument to popularity (argumentum ad populum) is a fallacy of irrelevance. The number of people who believe a particular proposition does not determine whether it is true or not. A proposition stands or falls on the evidence that supports it, no matter how many people believe it (or disbelieve it, for that matter).