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We propose a scenario where the saxion dominates the energy density of theUniverse and reheats the standard model sector via the dilatonic coupling,while its axionic partner contributes to dark matter decaying into photons viathe same operator in supersymmetry. Interestingly, for the axion mass $m_a\simeq 7$keV and the decay constant $f_a \simeq 10^{14-15}$GeV, the recentlydiscovered X-ray line at $3.5$keV in the XMM Newton X-ray observatory data canbe explained. We discuss various cosmological aspects of the $7$keV axion darkmatter such as the production of axion dark matter, the saxion decay process,hot dark matter and isocurvature constraints on the axion dark matter, and thepossible baryogenesis scenarios.

The 7 keV axion dark matter and the X-ray line signal