The word "un-marked" refers to something that has not been identified or labeled. Its opposite, the antonym, would be a word that describes something that has been identified or labeled. Some possible antonyms for "un-marked" include "marked," "notable," "distinguished," "identified," "labeled," "branded," "tagged," "named," "recgonizable," "characterized," "designated," "signaled," "emblazoned," "inscribed," and "engraved." Depending on the context, each of these words could be used to express the opposite meaning of "un-marked." For example, a marked trail is one that has clear signs and markers, while a distinguished person is one who stands out for their achievements or reputation.