When something "becomes fact," it means that it has been verified or proven to be true. Other synonyms for this phrase include "confirmed," "established," "validated," and "authenticated." These words all imply that there has been some level of investigation, testing, or research to establish the veracity of the claim. Other synonyms that carry a similar tone of certainty include "proven," "verifiable," and "undeniable." These words suggest that there is little room for doubt or debate about the truthfulness of the statement. Whatever word you use to describe the process of something becoming fact, it should carry a sense of authority and conviction.