When you Google search this, that's just what the definition comes out to. All the synonyms are of an idea, premise, presupposition, proposition, etc.
The antonym is fact.
The first definition when you Google the word theory is exactly what I and everyone else here has been explaining to you all thread; a theory is a framework for explaining a set of facts.
But again, the conversation here has nothing to do with how it is used most often colloquially which is what dictionaries will moreso reflect. The question here is how the word is used in science, because "the theory of evolution" is a scientific term.
This is very much like the word guilty, which in any normal context just means "he did it" but in a courtroom the word guilty specifically means "proven beyond a reasonable doubt by a unanimous jury verdict". Same word, but in a courtroom it is a much higher standard.
Theory means just that but in the world of professionals who practice and adhere to the principals of science.
any form of reasoning based on a subset of facts to come to a conclusion doesn't necessarily make the conclusion fact
All men are mortal.
Socrates is a man.
If both of these sentences are facts, wouldn't it follow that it is also a fact that Socrates is mortal?
It stands to reason that if evolution was fact, many religions would be shut down by now.
Most modern religions are based on the idea that 2,000 years ago a man rose from the dead. You can't possibly believe any scientific discovery will change the way religious people think.
A wise man once said "you can't reason a person out of a position they didn't reason themself into"
