Answer
The statement, "that is just a theory" is used in an attempt to convey that there isn't any scientific proof that backs up the statement. This goes against the context of the scientific theory as these theories are often tested more than once.
Work Step by Step
The statement, "that is just a theory" is used in an attempt to convey that there isn't any scientific proof that backs up the statement. This goes against the context of the scientific theory as these theories are often tested more than once.