I'd say that "deep space loneliness" counts as "dark".
I see/hear dark ambient not just as horror soundtracks or edgy sounds
but as the unseen, the mysterious, the "darkness", unsettling emotions,
uncertainty, lonely spaces, deep space, deep ocean, deep desert,
high mountain, dark forest, strange magics and discoveries, exploration
outside of your realm of safety, the vaguries of the night and of the universe...