matter has 7 distinct synonyms spread across 3 of its 7 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.
some situation or event that is thought about
“he kept drifting off the topic”
that which has mass and occupies space
“physicists study both the nature of matter and the forces which govern it”
(used with negation) having consequence
“they were friends and it was no matter who won the games”
written works (especially in books or magazines)
“he always took some reading matter with him on the plane”
Synonyms, antonyms and the broader and narrower relations for matter come from Princeton University’s WordNet. Full sources and licences.