Hmmm not sure what you mean with 'dark', but if you're looking for brooding, melancholy orchestral music then I'd recommend 'Memoryhouse' and 'Blue Notebooks' by Max Richter. Not a lot of 'happy' going on there. His music has a strong cinematic feel to it.
Also check out the instrumentals on the Cradle of Filth albums.
Not very sure here either about the concept of "dark" but the first thing that came to mind were the Ligeti's choral works. Pay a listen to the samples at Amazon
Those are the a capella works, other nice work by Ligeti with some obscure music is his Requiem. IIRC some part of it was used on the 2001 Space Odyssey film soundtrack.
For modern, dark choral works, be sure to check out Alfred Schnittke's Psalm of Repentance and Minnesang. Schnittke has a unique way of handling choral textures: he turns sounds into tentacles of swirling vortices that grip you and draw you into its gloomy core.
I also suspect a lot of Philip Glass soundtracks will pretty much hit a home-run with you. Dark & melancholic. Personal favorites are:
The Hours OST
Notes On A Scandal OST
The Illusionist OST
Dracula (performed by Kronos Quartet)