What the word covers now
In present-day English, dark is first of all a word for missing light: a room with the lamps off, a sky after sunset, a colour that reflects little brightness. It also describes hair or complexion that is brown or black rather than fair, a mood that is bleak, a plot that is hidden, and a stretch of history treated as barren of learning. As a noun it names the condition itself, as in after dark, or the state of not seeing. Later it named the state of not knowing.
That spread is not a modern pile-up of slang. The physical sense is the oldest layer. What changed is how far speakers pushed the same idea into secrecy, ignorance, colour, and character, and how the vowels in the written form moved.
Inherited, not borrowed
The origin of the word dark is a native Germanic inheritance, not a loan from French or Latin. Old English already used deorc, also spelled dearc. The adjective meant without light, especially at night, and also obscure or gloomy. It could already be figurative: cheerless, sinister, wicked. Middle English usually wrote derk, with a scatter of other spellings such as deork, dork, durk, and later dirk. The noun, recorded from the early thirteenth century in the sense of night-time or absence of light, is built from the adjective.
The adjective is in English before the year 1000, which is the expected picture for a core West Germanic word. It was already in the language while English was still a set of Old English dialects, not a later import waiting for Norman clerks.
A West Germanic stem with a thin family
Behind deorc stands a reconstructed West Germanic adjective *derk, with senses that cover both dark and dirty. Comparative dictionaries reconstruct Proto-Germanic *derkaz or *derka-. The same Germanic base is usually linked to Old High German tarchanjan (also terkanen, tarkenen), a verb meaning to hide or conceal, and to Middle High German terken, to darken or hide. Scots still has derk. The extinct Yola dialect of County Wexford had dhourk and durk.
What is absent matters as much as what is attested. There is no secure match in Old Norse or Gothic for this particular stem. The ordinary Old Norse word for darkness is myrkr, which English later took up on a different path as murky. Icelandic dökkr looks similar at a glance, but it belongs with another Germanic family, the one that also yields English dusk. So dark is a common English word with an unusually narrow set of near relations: West Germanic first, then English.
Deeper than Germanic, the trail is disputed. Some reconstructions propose a Proto-Indo-European *dʰerg- meaning to dim or darken, perhaps from an older *dʰer- associated with dull or dirty things. Tentative outside matches that have been offered include Tocharian words for cloud, Old Irish derg meaning red, Lithuanian dė́rgti for sleety or dirty weather, and a Balto-Slavic word for bad weather. None of these links is settled. Standard etymological notices still mark the ultimate source as uncertain. That is the honest stop: the English word is native, the West Germanic reconstruction is reasonable, and the Indo-European story remains a hypothesis.
Morphology does not add a hidden prefix. Dark is a simple adjective, not a transparent English compound. Related forms grew beside it: Old English deorcnysse for darkness, and deorcian, which became the verb darken.
How the senses moved
The oldest core is physical night and dimness. Already in Old English the figurative overlay was in place: gloom of mood, moral stain, things hard to see through. In Middle English the adjective covers unlit rooms and graves, lowering weather, dim heavenly bodies, and dull colour. Use for hues that do not radiate or reflect much light is in place by the late fourteenth century; so is the sense concealed or secret. Complexion described as not fair appears in the early fourteenth century. Early thirteenth-century English already uses the word for what is obscure or not easily understood.
Later centuries add finer shades rather than a new root. Sullen or sad as a quality of a look or mood is recorded from the 1590s. In the dark first meant in concealment, from the late fourteenth century, and only later, from the 1670s, in ignorance. Dark Ages as a label for a benighted stretch of history is attested by 1739; the narrower schoolbook sense, the early medieval centuries after Rome, is nineteenth-century. Dark horse, a competitor whose form is unknown, comes out of racing in the 1830s and was in American political talk by 1842. Theater slang for a house that is closed dates from 1916.
None of these later uses required a new etymology. They keep exploiting the same cluster: lack of light, lack of visibility, lack of knowledge.
The vowel that shifted in the fifteenth century
A concrete remnant of the Middle English form sits in the spelling history. The change from derk to dark is a fifteenth-century movement of the vowel in -er- before a consonant, the same pattern that turned deorling into darling, feorr into far, and heorot into hart. Scribes wrote both derk and dark for a time; the a form won.
If one fact is worth keeping, it is the isolation. English never replaced this West Germanic adjective with a Romance or Scandinavian twin, even though its farther ancestry is still hard to see.