Very early in the history of X-ray astronomy was discovered in 1962 by Riccardo Giacconi and employees a galactic X-ray background. The radiation is isotropic at energies of 2 keV and there is no correlation to galactic sources. X-ray brightness is proportional to the mass of a volume of stars included therein.
It is believed that the X-ray background is not a real background, but arises from the superposition of many individual sources. In the energy 1-10 keV could by ROSAT, XMM -Newton and Chandra about 80-90 % of the X-ray background into individual sources - mainly active galactic nuclei - have been resolved, at higher energies it is still pending.