dunkel
dark
With little or no light; dark.
Mach bitte das Licht an, es ist schon ganz dunkel im Zimmer.
Please turn on the light; it's already quite dark in the room.

dark
Loading your saved state.
dark
With little or no light; dark.
Mach bitte das Licht an, es ist schon ganz dunkel im Zimmer.
Please turn on the light; it's already quite dark in the room.

The adjective 'dunkel' (dark) is often used in contrast to its opposite 'hell' (light/bright). This applies to light and colors as well as to figurative meanings such as sounds.
Im Winter wird es früh dunkel und spät hell.
In winter, it gets dark early and light late.
To describe a color as a dark variant, 'dunkel' (dark) is simply placed before the color adjective. It then forms a new compound adjective.
Sie trägt am liebsten dunkelgrüne Kleidung.
She prefers wearing dark green clothing.
In the figurative meaning 'geheimnisvoll' (mysterious) or 'zwielichtig' (shady), 'dunkel' (dark) is often combined with nouns such as 'Vergangenheit' (past), 'Geheimnis' (secret), 'Ahnung' (premonition), or 'Kapitel' (chapter).
Er hat eine dunkle Ahnung, dass etwas nicht stimmt.
He has a dark suspicion that something is wrong.
When 'dunkel' (dark/deep) describes a sound, it means a low, resonant tone. It is used almost exclusively for voices or musical instruments.
Das Cello erzeugt einen warmen, dunklen Klang.
The cello produces a warm, deep sound.
The expression 'sich dunkel an etwas erinnern' (to vaguely remember something) is a fixed combination. It means that the memory is only vague and imprecise.
Ich kann mich nur dunkel an das Gesicht des Mannes erinnern.
I can only vaguely remember the man's face.
clear
'klar' and 'dunkel' can form a contrast when it comes to understanding or knowledge. 'klar' then means that something is unambiguous and easy to understand. In this context, 'dunkel' means that something is unclear, mysterious, or only vague. For example, you say that a memory is 'dunkel' if you do not remember it exactly. 'dunkel' is also possible for light or colors, but that is not the relevant opposite of 'klar' here.

Ich erinnere mich nur dunkel an dieses Gespräch.
I only vaguely remember that conversation.
Seine Antwort war kurz, aber ganz klar.
His answer was short, but completely clear.