dark

dark often used figuratively to mean unenlightened or ominous.
Dark Ages the period in western Europe between the fall of the Roman Empire and the high Middle Ages, c.500–1100 ad, during which Germanic tribes swept through Europe and North Africa, often attacking and destroying towns and settlements. It is traditionally viewed as being a time of unenlightenment, though scholarship was kept alive in the monasteries and learning was encouraged at the courts of Charlemagne and Alfred the Great.
the dark and bloody ground a name for Kentucky, popularized by the American poet Theodore O'Hara (1820–67) in the poem ‘The Bivouac of the Dead’ (1847). The phrase is sometimes said to be the meaning of Kentucky as an Indian term, but this has been questioned; alternatively, it is said to derive from a warning given by a Cherokee chief in the late 18th century, that the land was already ‘a bloody ground’ from earlier hunting and fighting, and that it would be dark for prospective settlers.
the Dark Continent a name given to Africa at a time when it was little known to Europeans, first recorded in H. M. Stanley's Through the Dark Continent (1878).
dark horse a person about whom little is known, especially someone whose abilities or potential for success is concealed. The term is originally racing slang, denoting a horse about whose racing powers little is known, and is first recorded in the mid 19th century.
Dark Lady the woman to whom a number of Shakespeare's sonnets were written; the Elizabethan scholar A. L. Rowse suggested that she may have been Emilia Lanier (1569–1645), but she has never been certainly identified; an alternative name is that of Mary Fitton (1578–1647).
dark matter in some cosmological theories, non-luminous material which is postulated to exist in space and which could take either of two forms: weakly interacting particles (cold dark matter) or high-energy randomly moving particles created soon after the big bang (hot dark matter).
dark night of the soul a period of spiritual desolation suffered by a mystic in which all sense of consolation is removed. ‘Faith, the dark night of the soul’ appears in David Lewis's 1864 translation of the Complete Works of St John of the Cross (1542–91), as the translator's chapter heading for the poem Noche oscura [Dark night]. The expression was later popularized by F. Scott Fitzgerald in the form, ‘In a real dark night of the soul it is always three o'clock in the morning.’
it was a dark and stormy night one variant of an opening line intended to convey a threatening and doom-laden atmosphere; in this form used by the novelist Edward Bulwer-Lytton (1803–73) in his novel Paul Clifford (1830).

See also all cats are grey in the dark at cat, cold dark matter, darkest, a leap in the dark, a shot in the dark.