  Dark matter might be made of "warm" particles, such as sterile neutrinos in
the keV mass range, which can decay into photons through mixing and are
consequently detectable by X-ray telescopes. Axionlike particles (ALPs) are
detectable by X-ray telescopes too when coupled to standard model particles and
decay into photons in the keV range. Both particles could explain the
unidentified 3.5 keV line and, interestingly, XENON1T observed an excess of
electron recoil events most prominent at 2-3 keV. One explanation could be an
ALPs origin, which is not yet excluded by X-ray constraints in an anomaly-free
symmetry model in which the photon production is suppressed. We study the
diffuse emission coming from the Galactic halo, and calculate the sensitivity
of all-sky X-ray survey performed by eROSITA to identify a sterile neutrino or
ALP dark matter. We estimate bounds on the mixing angle of the sterile
neutrinos and coupling strength of the ALPs. After four years of data-taking by
eROSITA, we expect to set stringent constraints, and in particular, we expect
to firmly probe mixing angle $\sin^2(2\theta)$ up to nearly two orders
magnitude below the best-fit value for explaining the unidentified 3.5 keV
line. Moreover, with eROSITA, we will be able to probe the ALP parameter space
of couplings to photons and electrons, and potentially confirm an ALP origin of
the XENON1T excess.
