galaxy has 6 distinct synonyms spread across 2 of its 3 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 splendid assemblage (especially of famous people)
tufted evergreen perennial herb having spikes of tiny white flowers and glossy green round to heart-shaped leaves that become coppery to maroon or purplish in fall
Synonyms, antonyms and the broader and narrower relations for galaxy come from Princeton University’s WordNet. Full sources and licences.
(astronomy) a collection of star systems; any of the billions of systems each having many stars and nebulae and dust
“`extragalactic nebula' is a former name for `galaxy'”