  An excess $\gamma$-ray signal toward the outer halo of M31 has recently been
reported. Although other explanations are plausible, the possibility that it
arises from dark matter (DM) is valid. In this work we interpret the excess in
the framework of DM annihilation, using as our representative case WIMP DM
annihilating to bottom quarks, and we perform a detailed study of the
systematic uncertainty in the $J$-factor for the M31 field. We find that the
signal favors a DM particle with a mass of $\sim$46-73 GeV. While the mass is
well constrained, the systematic uncertainty in the cross-section spans 2.5
orders of magnitude, ranging from $\sim$8$\times 10^{-27}-4 \times 10^{-24} \
\mathrm{cm^3 \ s^{-1}}$. This high uncertainty is due to two main factors,
namely, an uncertainty in the substructure nature and geometry of the DM halos
for both M31 and the Milky Way (MW), and correspondingly, an uncertainty in the
contribution to the signal from the MW's DM halo along the line of sight.
However, under the conditions that the minimum subhalo mass is $\lesssim
10^{-6} \ M_\odot$ and the actual contribution from the MW's DM halo along the
line of sight is at least $\sim$30$\%$ of its total value, we show that there
is a large overlap with the DM interpretations of both the Galactic center (GC)
excess and the antiproton excess, while also being compatible with the limits
for the MW dwarf spheroidals. More generally, we summarize the results from
numerous complementary DM searches in the energy range 10 GeV $-$ 300 GeV
corresponding to the GC excess and identify a region in parameter space that
still remains viable for discovery of the DM particle.
