Thanks LindaLou and LinearAQ.

Quote
If there is the possibility that X exists, there may never be any evidence uncovered until someone goes to look for it.
Here's an example of invalidation:

Obserrvation: No living Ivory Billed Woodpeckers have been observed for over 50 years, longer than any individual Ivory Billed Woodpecker is known to live.

Theory: Ivory Billed Woodpeckers have gone extinct and no longer exist.

Falsification test: find an Ivory Billed Woodpecker

http://www.birds.cornell.edu/ivory/more_info/

Enjoy.