What happens if you remove traditional melody and propulsive rhythms from instrumental music?
Whether stemming from minimalist composition (Pauline Oliveros or Steve Reich, for example) or early experiments in electronic music (Morton Subotnick or, in the early 1970s, Klaus Schulze) the result is music that has a quality of filling the space without dominating it.
Brian Eno’s Ambient 1: Music For Airports was perhaps the most significant work in popularising minimalist ambient music. His idea was an album you could immerse yourself in… or ignore. That background/foreground attribute is what characterises ambient music from the 70s onwards. Other important names include Paul Horn (his "Inside" albums), Steve Roach and Max Richter.
Meanwhile, other artists were exploring adaptations of ambient music to create downtempo, chillout, and music recorded specifically for relaxation.