Antimatter aren't really ambient, more like trip hop mixed with dark rock, almost like newer 3rd and the Mortal.
If you like dark ambient like Raison d'être, definitely get all the albums from that record label, Cold Meat Industry. Also, Thrussel from Snog's side project Black Lung is mandatory. Sephiroth is also quite good dark ambient, and you should check out Lustmord too. I haven't heard that much dark ambient recently, as I got quickly bored of it, but back in the day these artists interested me somewhat. For a final note, the absolute best dark ambient artist was Delerium. Now, Delerium makes sort of ambient pop music like Enigma, but in the 80's and early 90's they made the best dark ambient. Get these albums: Stone Tower, Spheres, and Spheres II. Also, Spiritual Archives is rather chilling.