X-ray background
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The observed X-ray background is thought to result from, at the "soft" end (below 0.3 keV), Galactic X-ray emission (the "galactic" X-ray background), and, at the "hard" end (above 0.3keV), from a combination of many unresolved X-ray sources outside of the Milky Way (the "cosmic" X-ray background).
The X-ray background is occulted by the dark side of the Moon.
The galactic X-ray background is produced largely by emission from hot gas in the Local Bubble within 100 parsecs of the Sun.
Deep surveys with X-ray telescopes, such as the Chandra X-ray Observatory, have demonstrated that around 80% of the cosmic X-ray background is due to resolved extra-galactic X-ray sources, the bulk of which are unobscured ("type-1") and obscured ("type-2") active galactic nucleii (AGN).