  A signal from decaying dark matter (DM) can be unambiguously distinguished
from spectral features of astrophysical or instrumental origin by studying its
spatial distribution. We demonstrate this approach by examining the recent
claim of 0912.0552 regarding the possible DM origin of the 2.5 keV line in
Chandra observations of the Milky Way satellite known as Willman 1. Our
conservative strategy is to adopt a relatively large dark mass for Willman 1
and relatively small dark masses for the comparison objects. We analyze
archival observations by XMM-Newton of M31 and Fornax dwarf spheroidal galaxy
(dSph) and Chandra observations of Sculptor dSph. By performing a conservative
analysis of X-ray spectra, we show the absence of a DM decay line with
parameters consistent with those of 0912.0552. For M31, the observations of the
regions between 10 and 20 kpc from the center, where the uncertainties in the
DM distribution are minimal, make a strong exclusion at the level above
10sigma. The minimal estimate for the amount of DM in the central 40 kpc of M31
is provided by the model of 0912.4133, assuming the stellar disk's mass to
light ratio ~8 and almost constant DM density within a core of 28 kpc. Even in
this case one gets an exclusion at 5.7sigma from central region of M31 whereas
modeling all processed data from M31 and Fornax produces more than 14sigma
exclusion. Therefore, despite possible systematic uncertainties, we exclude the
possibility that the spectral feature at ~2.5 keV found in 0912.0552 is a DM
decay line. We conclude, however, that the search for DM decay line, although
demanding prolonged observations of well-studied dSphs, M31 outskirts and other
similar objects, is rather promising, as the nature of a possible signal can be
checked. An (expected) non-observation of a DM decay signal in the planned
observations of Willman 1 should not discourage further dedicated observations.
