Origins
The word 'dark' descends from Old English 'deorc' (dark, obscure, gloomy, without light), from Proto-Germanic *derkaz (dark, hidden). Its further etymology is uncertain and debated, which gives the word a fittingly mysterious quality — the word for obscurity is itself obscure.
Unlike most basic English vocabulary, 'dark' has no clear cognates outside the West Germanic branch of the Indo-European family. Old High German 'tarchanjan' (to hide, to conceal) and Middle High German 'terken' (to soil, to make dirty) may be related, but the connection is not certain. There is no Old Norse cognate, no Gothic cognate, and no parallel in any non-Germanic Indo-European language. This extreme isolation is unusual for a word denoting such a fundamental concept — most languages share ancient words for basic sensory experiences like light, darkness, heat, and cold across the family.
Several explanations have been proposed for this isolation. One possibility is that *derkaz was a late Proto-Germanic innovation, coined after the branch had already separated from the rest of the Indo-European family. Another is that it was borrowed from a pre-Indo-European substrate language — the languages spoken in northern Europe before the arrival of Indo-European speakers, which left scattered traces in the Germanic vocabulary. A third possibility, sometimes suggested, connects *derkaz to PIE *dʰerg- (to make dirty, to darken), but this root is itself poorly attested.
Old English Period
The Old English adjective 'deorc' was used both literally (without light, dim) and figuratively (gloomy, evil, mysterious, difficult to understand). The figurative senses reflect the widespread cross-linguistic metaphor in which darkness represents ignorance, evil, and concealment, while light represents knowledge, goodness, and revelation. This metaphor, often called the Light-Darkness metaphor, is not unique to Indo-European languages — it appears in virtually every human culture, likely because vision depends on light and most human beings experience darkness as disorienting and threatening.
The noun 'dark' (the absence of light, as in 'afraid of the dark' or 'in the dark') developed from the adjective in Middle English. The phrase 'in the dark' (ignorant, uninformed) dates from the 1670s. 'Dark horse' (an unknown competitor who unexpectedly wins) originated in horse racing in the 1830s — Benjamin Disraeli used the term in his 1831 novel 'The Young Duke.' 'The Dark Ages' as a term for the early medieval period was coined by Petrarch in the 14th century, who saw his own era as a return to the 'light' of classical civilization after centuries of cultural 'darkness.'
The compound 'darkling' (in the dark, growing dark) is a poetic archaism that survives primarily in literary contexts. Matthew Arnold's 'Dover Beach' (1867) contains the famous line 'we are here as on a darkling plain,' and Keats wrote of a nightingale singing 'in some melodious plot / Of beechen green, and shadows numberless, / Darkling I listen.' The suffix '-ling' here is an adverbial ending (as in 'sideling'), not the diminutive '-ling' of 'duckling' or 'darling.'