I'll consider the "huge soundscapes and a really gloomy atmosphere, that are better appreciated in a dark room with headphones" part. I'm not really into that kind of stuff, but this sounds like you might want to try "doom/jazz" band The Mount Fuji Doomjazz Corporation -
Roadburn.
An adequate review :
To a larger extent than in their previous work, the doom now considerably outweights the jazz, a progression from fin de siècle brooding to post-apocalyptic devastation. Melody, which has always played a significantly poignant role in TMFDC’s music, here becomes downright painful to hear, woefully frail within the dense, often relentless walls of noise that form Roadburn‘s blasted bedrock. Imagine Ad Reinhard’s black paintings texture-mapped over shards of glass rotating in deep space & you get some idea of the endlessness & remoteness of this music. When points of stability do emerge, they each progress like a grimy industrial cortège, like shuffling towards the edge of a cliff. i appreciate that these kinds of allusion may not seem like the typical language of recommendation, but Roadburn—like so much of TMFDC’s music—is blisteringly beautiful, offering what’s perhaps best described as ‘antijoy’. Throughout the first two movements (accounting for more than half the work’s duration) melody is allowed no inroads at all, amounting to nothing more than absent-minded murmuring & barely-glimpsed afterthought. It’s only with the third movement that the trombone manages to make its way to the surface, forcing a re-colouration of the music that, notwithstanding the noise & pounding pulse, bestows on this wasteland of sound a kind of grandeur, even majesty; not surprisingly, however, it’s neither subtle nor restrained, expanding the third movement into a drawn-out blare that’s positively riddled with emotional subtext. & in no time it’s gone, the short, closing fourth movement swollen from within by a churning fog, obliterating all trace of anything identifiable.
The previous albums have a little bit more melody in there. In their last one, it's pretty much just a gloomy and bleak soundscape.
You can also try Kreng -
Works For Abattoir Ferme 2007 - 2011. It's music for theater pieces and videos.
This is one of the least bleak example :
https://www.youtube.com/watch?v=3mOziiftT9QSomeone said Stravinsky's Rite of spring, but I don't think it's dark music. It's exhilarating and primitive and strange sometimes, not dark. But you might find plenty of really dark music in "serious music" after 1950, like Raphael Cendo's "Introduction aux ténèbres" (it literally means "introduction to darkness"), a lot of Penderecki's, Sciarrino's, Birtwistle's and Crumb's stuff, and plenty others. I might think of a list if someone is interested, but all of that would way more darker, scarier and harder than anything that have been notified in this thread.