No one has shown that Mars was not created by the aliens. Therefore, Mars is actually created by the aliens. Which fallacy is involved in the above statement?

Concluding a claim true merely because it has not been disproved is the appeal to ignorance.

No one has shown that Mars was not created by aliens.

From this lack of disproof the creation by aliens is asserted as fact.

Absence of evidence against a claim is not evidence in favour of it.

Treating the unproven-false as thereby true is argumentum ad ignorantiam.