Historically, the IGM motivated the search for FUV background which in turn led to a number of experiments and missions. Decades later the focus shifted to FUV as the primary heating and ionizing agent of the atomic phases (warm and cold neutral medium). The current view is that the diffuse FUV emission, at high latitudes, has three components: FUV light from hot stars in the Galactic plane reflected by dust grains (diffuse galactic light or DGL), FUV from other galaxies (extra-galactic background light, EBL) and a component of unknown origin. During the eighties, there was some discussion that decaying dark matter particles produced FUV radiation. In my talk I systematically investigate production of FUV photons by the Galactic Hot Ionized Medium (line emission) and two photon emission from the Warm Ionized Medium, the general class of low velocity shocks and from Lyman fluorescence in the Solar System (the interplanetary medium and the exosphere of Earth). I conclude that two thirds to perhaps all of the third component can be explained by the sum of the processes listed above.
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