Officially it is known as 'argument from ignorance.'
Absence of evidence is not proof.
Or Absence of evidence is NOT evidence of absence.
Or put more simply, just cuz you can't find it doesn't prove it aint there or duzzint exist.
Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good ...