unknown has 11 distinct synonyms spread across 6 of its 8 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is known.
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.
being or having an unknown or unnamed source
“a poem by an unknown author”
an unknown and unexplored region
“they came like angels out the unknown”
1 of the 8 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for unknown.
Synonyms, antonyms and the broader and narrower relations for unknown come from Princeton University’s WordNet. Full sources and licences.