Difficult to understand; obscure: stories that are large in scope and dark in substance.
Concealed or secret; mysterious: "the dark mysteries of Africa and the fabled wonders of the East" (W. Bruce Lincoln).
Lacking enlightenment, knowledge, or culture: a dark age in the history of education.
Exhibiting or stemming from evil characteristics or forces; sinister: "churned up dark undercurrents of ethnic and religious hostility" (Peter Maas).
Being or characterized by morbid or grimly satiric humor.
Having richness or depth: a dark, melancholy vocal tone.