  Recent studies of M31, the Galactic centre, and galaxy clusters have made
tentative detections of an X-ray line at ~3.5 keV that could be produced by
decaying dark matter. We use high resolution simulations of the Aquarius
project to predict the likely amplitude of the X-ray decay flux observed in the
GC relative to that observed in M31, and also of the GC relative to other parts
of the Milky Way halo and to dwarf spheroidal galaxies. We show that the
reported detections from M31 and Andromeda are compatible with each other, and
with upper limits arising from high galactic latitude observations, and imply a
decay time {\tau} ~10^28 seconds. We argue that this interpretation can be
tested with deep observations of dwarf spheroidal galaxies: in 95 per cent of
our mock observations, a 1.3 Msec pointed observation of Draco with XMM-Newton
will enable us to discover or rule out at the 3{\sigma} level an X-ray feature
from dark matter decay at 3.5 keV, for decay times {\tau} < 0.8x10^28 sec.
