  We discuss the potential of Athena X-ray telescope, in particular of its
X-ray Integral Field Unit (X-IFU), for detection of the signal from the
light-weight decaying dark matter with mass in the keV range. We show that high
energy resolution and large collection area of X-IFU will provide an
improvement of sensitivity which will be sufficient for the full test of the
neutrino Minimal extension of the Standard Model (nuMSM). Search for the narrow
spectral line produced by the decay of the dark matter sterile neutrino in the
spectra of dwarf spheroidal galaxies with X-IFU will explore the whole allowed
range masses and mixing angles of the nuMSM lightest sterile neutrino and in
this way either to find the dark matter signal or rule out the nuMSM model.
