X-ray emission covering the entire sky, discovered by rocket flights in the early 1960s. The ‘soft’ (low-energy) background was originally thought to come from hot gas in the local bubble but is now believed to be mostly due to processes within the heliosphere. In contrast, the ‘hard’ (high-energy) background mostly comes from discrete extragalactic sources, notably active galaxies, quasars, and X-ray binaries in other galaxies.