NASA released sonifications of the Perseus cluster a few years back and what most people remember is the impossibly low B-flat — a tone fifty-seven octaves below middle C, theoretically emitted by the black hole at the centre. These 15 black hole sound effects sit in that lineage rather than in pure sci-fi fantasy: a deep gravitational drone with sub-bass weight, a slow rumble that builds and falls over twenty seconds, a steady cosmic hum suitable for room-tone under dialogue, and dark space backgrounds with almost no high frequency content at all.
Sci-fi trailers use the deep drone under the studio logo because it makes the room feel three sizes bigger before the title card lands. Documentary work on astrophysics pairs the gravitational hum with narration and lets the silence do the lifting. Game cinematics for cosmic-horror titles loop the long backgrounds for entire chapters. Pull whatever fits the scale — free to download, no signup, no licence to chase.