There are two different usages of the word "theory". In the general community, the word is used to mean something like "speculative guess", as you say. In science, though, a theory is a well-established explanation of a scientific fact. Thus we have Newton's theory of gravity, Darwin's theory of evolution etc. A speculative guess in science is usually called a "tentative hypothesis"; mathematicians like the word "conjecture".
A scientific theory isn't fact either though. Theories get updated all the time as new information is found. Scientific theories are to the best as we currently know. But yes, scientific theories hold a bit more weight than the average one.
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