  An interesting result in particle astrophysics is the recent detection of an
unexplained 3.5 keV line from galaxy clusters. A promising model, which can
explain the morphology of the signal and its non-observation in dwarf
spheroidal galaxies, involves a 7 keV dark matter particle decaying into a pair
of ultra-light axions that convert into photons in the magnetic field of the
clusters. Given that light axions emerge naturally in 4D string vacua, in this
paper we present a microscopic realisation of this model within the framework
of type IIB flux compactifications. Dark matter is a local closed string axion
which develops a tiny mass due to subdominant poly-instanton corrections to the
superpotential and couples via kinetic mixing to an almost massless open string
axion living on a D3-brane at a singularity. The interaction of this
ultra-light axion with photons is induced by U(1) kinetic mixing. After
describing the Calabi-Yau geometry and the brane set-up, we discuss in depth
moduli stabilisation, the resulting mass spectrum and the strength of all
relevant couplings.
