I think you are misunderstanding the use of the word theory. I will say I cannot account for how others on this board are using it but it seems that you believe the use of theory implies that there can be some question or doubt as to whether something is true. That is not how that word is always used.
Definition of theory
1: a
plausible or scientifically acceptable general principle or body of principles offered to explain phenomenathe wave theory of light
2a: a belief, policy, or procedure proposed or followed as the basis of actionher method is based on the theory that all children want to learn
b: an ideal or hypothetical set of facts, principles, or circumstances —often used in the phrase
in theoryin theory, we have always advocated freedom for all
3a: a hypothesis assumed for the sake of argument or investigation
b: an unproved assumption :
CONJECTURE
c: a body of
theorems presenting a concise systematic view of a subjecttheory of equations
4: the general or abstract
principles of a body of fact, a science, or an artmusic theory
5: abstract thought :
SPECULATION
6: the analysis of a set of facts in their relation to one another