Depending on the nature of the matter the standard of proof may require that a fact be proved to be "more likely than not", that is there is barely more evidence for the fact than against; established by a preponderance of the evidence; or true beyond a reasonable doubt.
There are various types of facts, such as proverbial facts-for instance that dogs are man's best friend which is more of an opinion really.
Facts which aren't can be found on Misconceptions and disputed facts