dark has 27 distinct synonyms spread across 10 of its 15 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 light.
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.
devoid of or deficient in light or brightness; shadowed or black
“sitting in a dark corner”
(used of color) having a dark hue
“dark green”
marked by difficulty of style or expression
“much that was dark is now quite clear to me”
absence of light or illumination
the time after sunset and before sunrise while it is dark outside
an unenlightened state
“he was in the dark concerning their intentions”
3 of the 15 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for dark.
Synonyms, antonyms and the broader and narrower relations for dark come from Princeton University’s WordNet. Full sources and licences.