  We propose an X-ray mission called Xenia to search for decaying superweakly
interacting Dark Matter particles (super-WIMP) with a mass in the keV range.
The mission and its observation plan are capable of providing a major break
through in our understanding of the nature of Dark Matter (DM). It will
confirm, or reject, predictions of a number of particle physics models by
increasing the sensitivity of the search for decaying DM by about two orders of
magnitude through a wide-field imaging X-ray spectrometer in combination with a
dedicated observation program.
  The proposed mission will provide unique limits on the mixing angle and mass
of neutral leptons, right handed partners of neutrinos, which are important
Dark Matter candidates. The existence of these particles is strongly motivated
by observed neutrino flavor oscillations and the problem of baryon asymmetry of
the Universe.
  In super-WIMP models, the details of the formation of the cosmic web are
different from those of LambdaCDM. The proposed mission will, in addition to
the search for decaying Dark Matter, provide crucial insight into the nature of
DM by studying the structure of the "cosmic web". This will be done by
searching for missing baryons in emission, and by using gamma-ray bursts as
backlight to observe the warm-hot intergalactic media in absorption.
