The main German word for “darkness” is “Dunkelheit” (f.), from the adjective “dunkel”, meaning “dark”.
“Düsterkeit” (f.), “Düsterheit” (f.) and (more rarely) “Düsternis” (f.) can be used in the sense of “gloom” and “dreariness”. The adjective is either “duster” or “düster”.
A somewhat more antiquated word for “darkness” is “Finsternis” (f.), which may be found in poetic and older literary works. Here the adjective is “finster”.
A related word is “zappenduster”, which essentially means “very/completely dark” and is also used to describe bleak-looking situations. This word is thought to describe the darkness following the “Zapfenstreich”, originally a musical signal indicating the beginning of curfew (or “military tattoo”).
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Examples
Wir müssen da sein bevor es dunkel wird. We must be there before dark.
Das ist eine dunkle Farbe. This is a dark color.
Seine Laune ist düster. His mood is gloomy.
Er nennt sich ‘Fürst der Finsternis’. He calls himself “Prince of Darkness”.
Das sind düstere Aussichten. That’s a bleak outlook.
Wenn du das studierst sieht es beruflich zappenduster aus. If you study that your professional future looks bleak.