The Mystery of the Cosmic Diffuse Ultraviolet Background Radiation
  The diffuse cosmic background radiation in the GALEX far ultraviolet (FUV,
1300 \AA\ - 1700 \AA) is deduced to originate only partially in the
dust-scattered radiation of FUV-emitting stars: the source of a substantial
fraction of the FUV background radiation remains a mystery. The radiation is
remarkably uniform at both far northern and far southern Galactic latitudes,
and it increases toward lower Galactic latitudes at all Galactic longitudes. We
examine speculation that it might be due to interaction of the dark matter with
the nuclei of the interstellar medium but we are unable to point to a plausible
mechanism for an effective interaction. We also explore the possibility that we
are seeing radiation from bright FUV-emitting stars scattering from a "second
population" of interstellar grains---grains that are small compared with FUV
wavelengths. Such grains are known to exist (Draine 2011) and they scatter with
very high albedo, with an isotropic scattering pattern. However, comparison
with the observed distribution (deduced from their $100\ \mu$m emission) of
grains at high Galactic latitudes shows no correlation between the grains'
location and the observed FUV emission. Our modeling of the FUV scattering by
small grains also shows that there must be remarkably few such "smaller" grains
at high Galactic latitudes, both North and South; this likely means simply that
there is very little interstellar dust of any kind at the Galactic poles, in
agreement with Perry & Johnston (1982). We also review our limited knowledge of
the cosmic diffuse background at ultraviolet wavelengths shortward of Lyman
$\alpha$---it could be that our "second component" of the diffuse
far-ultraviolet background persists shortward of the Lyman limit, and is the
cause of the re-ionization of the Universe (Kollmeier et al. 2014).
