There is an enormous difference between a fact - that has been observed and confirmed by various observers - and a plausible theory. There are many theories in physics, for example, some with many elements confirmed in numerous experiments, but still called theories.

When I read history books, authors often mix these two. As a reader, I don't know whether a given statement is a fact (an artifact was found, several contemporary authors described an event in a uniform way), or just an interpretation that seemed plausible in a given situation