  The Standard Model of particle physics can be extended to include sterile
(right-handed) neutrinos or axions to solve the dark matter problem. Depending
upon the mixing angle between active and sterile neutrinos, the latter have the
possibility to decay into monoenergetic active neutrinos and photons in the
keV-range while axions can couple to two photons. We have used data taken with
X-ray telescopes like e.g. XMM-Newton for the search of line emissions. We used
pointings with high exposures and expected dark matter column densities with
respect to the dark matter halo of the Milky Way. The posterior predictive
p-value analysis has been applied to locate parameter space regions which
favour additional emission lines. In addition, upper limits of the parameter
space of the models have been generated such that the preexisting limits have
been significantly improved.
