Its plenty dark imo, the beat is pretty powerful, maybe the kick could be a bit more prominent. Think the track would darken up a bit more if you added some fx/short vocals actually, it would help the atmosphere a bit.
Thiiiiiiing here is you need to keep on doing tunes. Try to make whole song structures (5 min long tunes or so) it will help A LOT in the mixdowns (volume overall).
On the other hand, how to 'harden' this? One way could be adding a lead, another way would be adding silences each 8 bars, but these are like the most common.
Keep at it man!! I'd like to see you posting here more often )