WordNet records no synonym and no antonym for fact. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a piece of information about circumstances that exist or events that have occurred
“first you must collect all the facts of the case”
Synonyms, antonyms and the broader and narrower relations for fact come from Princeton University’s WordNet. Full sources and licences.
a statement or assertion of verified information about something that is the case or has happened
“he supported his argument with an impressive array of facts”
an event known to have happened or something known to have existed
“your fears have no basis in fact”
a concept whose truth can be proved
“scientific hypotheses are not facts”