dark

(adjective): Having an absolute or (more often) relative lack of light.
(adjective): (of colour) Dull or deeper in hue; not bright or light.
(adjective): Hidden, secret, obscure.
(adjective): Without moral or spiritual light; sinister, malign.
(adjective): Conducive to hopelessness; depressing or bleak.
(adjective): Lacking progress in science or the arts; said of a time period.
(adjective): With emphasis placed on the unpleasant aspects of life; said of a work of fiction, a work of nonfiction presented in narrative form or a portion of either.

The room was too dark for reading.
my sister's hair is darker than mine; her skin grew dark with a suntan
a dark villain; a dark deed
the Great Depression was a dark time; the film was a dark psychological thriller
The ending of this book is rather dark.