hmm dark sounding, I think more than anything it has to be a few set of chords put together that create the dark mood but you can try the diminished scale as bonzotracker said, you can maybe try third inversion of minor triads. I wouldn't recommend adding anything other than a 6th to those triads, 7ths, 9ths etc make the chords sound... not particularly bright but a bit elegant and jazzy, they may come across as sad but not sure if they would fulfill the dark mood you are trying to get, you can try em out if you like. You might also want to check out the half diminished chord and maybe the minor - major 7 chord. A lot of these you might have to pay attention to their voicing though because they sound pretty dissonant.
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Music we need when language fails us, but we cannot remain silent.