Portishead are pretty dark, their recent stuff's more experimental but might be better to start with their first album to get into them.
Seeing Aphex twin brought up here reminded me of a guy I met who had seen him live. he said aphex played a backstreet boys tune mixed with drill whenever the room got too full
Try Monolake "Ghosts" and Venetian Snares "Doll Doll Doll"/"Find Candace" (ok those ones are more insane than dark but if you are searching for something a little bit extreme he is the man...)
I am Amon Tobins fan as well as The Future Sound Of London... the second one I love and have listened a lot. Dead Cities and Lifeforms albums... very imaginative music