Theoretically
supposition
The opposite of theory is typically practice. While theory refers to a set of principles or ideas used to explain something, practice involves the actual application or implementation of those principles.
A significant theory is one that can be supported by relevant material, in which the theory you are proposing is in fact a possibility.
theory
on paper
It always was a fact and to the best of my knowledge no-one ever doubted it.
Scientific theory is two words.
Hypothesis.
A "theory" is a scenario of what might be.
The word you are looking for is "hypothetical." It refers to something that is assumed or imagined but may not be true or real in practice.
The unviers is usually thought of to be a exposhon. But this is not a theory, it is irrefutale fact. One theory is that water cannot escape the unviers.
A clue is a datum - a fact. A "theory" tries to tell you what the fact means.