Is there a word for an idea similar to negative evidence?

Solution 1:

It sounds like conventional wisdom

the body of ideas or explanations generally accepted as true by the public or by experts in a field. Such ideas or explanations, though widely held, are unexamined.

Wikipedia

The term given (as a noun) can also apply

an assumed fact

Collins

Solution 2:

The term I would use would be default position, borrowed from wordprocessing, though the only example I have is from a Wordpress page.

The term “default position” refers to a belief (or lack of belief) that is preferable prior to debate or before any evidence is considered.

Solution 3:

Historians and the like call this tacit (or implicit) knowledge - things that were considered so obvious no-one ever bothered to write them down, but which may not be at all obvious centuries or millennia later.