Is appeal to ignorance a logical fallacy?

An appeal to ignorance (ignorance here meaning lack of evidence) is a type of informal logical fallacy.

It asserts that something must be true because it hasn’t been proven false—or that something must be false because it has not yet been proven true.

For example, “unicorns exist because there is no evidence that they don’t.” The appeal to ignorance is also called the burden of proof fallacy.