An informal fallacy in which something is assumed to be a fact because the contrary of it is not yet proven to be true is called

Assuming a claim is fact because its opposite is unproven is the appeal to ignorance.

Argumentum ad ignorantiam treats absence of disproof as if it were proof.

Not having shown something false does not thereby show it true.

The want of contrary evidence leaves the matter open, not settled.

This misuse of ignorance as evidence names the fallacy.