Beethoven's third and seventh both fit what you are looking for (listen to the second movements of both for dark and minor).
Brahms' 4th is another good one in that sort of idiom, as is his first.
Bruckner's 8th.
Mussorgsky's Night on Bald Mountain (This is fast ok? But I don't know anyone who doesn't like it doesn't like dark and minor key music anyway)
If you're feeling a bit adventurous and aren't afraid of stuff that isn't in 'normal' tonality, definitely listen to some Alfred Schnittke. Anything by him written from around the late 70s onwards. Recently I've been listening to his sixth symphony a lot; it is great.
Arvo Pärt's Tabula Rasa, or Cantus in Memoriam Benjamin Britten.
Avoid American composers at all costs, really. They tend to write more upbeat or whimsical sorts of things, or just wank around trying to impress each other. No real darkness, unless it's really affected. Western Europe is less upbeat, but there's stil a focus on academia (not that there's necessarily anything wrong with that) rather than stuff to mess you up inside.
If you want dark modern music, look to Eastern Europe - pretty much any Russian composer since Shostakovich, and ex-Eastern Bloc composers like Pärt, György Ligeti, Sophia Gubaidulina (hey check it out fellas women can write music too) or Henryk Górecki. It's pretty understandable why this is: American composers are typically from white, upper-middle class backgrounds and therefore have not (in general ok) really endured much in the way of hardship or personal strife. Russian composers not only went through the economic downfall of Communism but also dictatorial control over what music was acceptable. Composers disappeared if the state didn't like what they were doing.
Though on the other hand a lot of their music is more spiritual and uplifting (like most of Pärt's) so that can be something to watch out for - they either wrote dark stuff to express personal anguish or found solace in God. Schnittke did a kind of a mix of the two - his spiritual music is pretty damn depressing.
Anyway there are some ideas off the top of my head. There's a bit of a range of styles there so see what you like/don't like. If you don't like any of it say so and we'll try to think of some other stuff.
