  Primordial decays of string theory moduli at z \sim 10^{12} naturally
generate a dark radiation Cosmic Axion Background (CAB) with 0.1 - 1 keV
energies. This CAB can be detected through axion-photon conversion in
astrophysical magnetic fields to give quasi-thermal excesses in the extreme
ultraviolet and soft X-ray bands. Substantial and observable luminosities may
be generated even for axion-photon couplings \ll 10^{-11} GeV^{-1}. We propose
that axion-photon conversion may explain the observed excess emission of soft
X-rays from galaxy clusters, and may also contribute to the diffuse unresolved
cosmic X-ray background. We list a number of correlated predictions of the
scenario.
