Which informal logical fallacy is committed in the following argument? 'No one has shown that aliens did not create the crop circles, so the crop circles must have been made by aliens.'

The argument treats the absence of disproof as proof.

No one has shown that aliens did not make the crop circles.

From this lack of contrary evidence it concludes aliens must have made them.

Failure to disprove a claim is not evidence that the claim is true.

Inferring truth from mere absence of refutation is the appeal to ignorance.