Is it farther or further?
c. 1300, "to make dark;" late 14c., "to become dark," from dark (adj.) + -en (1). The more usual verb in Middle English was simply dark, as it is in Chaucer and Shakespeare, and darken did not predominate until 17c. The Anglo-Saxons also had a verb sweorcan meaning "to grow dark." To darken someone's door (usually with a negative) is attested from 1729.