  Recently, possible indications of line spectral features in the Fermi-LAT
photon spectrum towards the galactic center have been reported. If the distinct
line features arise from dark matter (DM) annihilation into $\gamma X
(X=\gamma, Z^{0} or h^{0})$, the corresponding annihilation cross-section is
unnaturally large for typical loop-induced radiative processes. On the other
hand, it is still too small to be responsible for the observed DM relic
density. We show that the mechanism of Sommerfeld enhancement with scalar
force-carrier can provide a simple solution to these puzzles. The possibly
large Sommerfeld enhancement of the cross-section for s-wave DM annihilation
can significantly reduce the required effective couplings between DM and
charged particles in typical loop diagrams. The DM particles necessarily
annihilate into scalar force-carriers through tree-level p-wave process, which
can dominate the total DM annihilation cross-section at freeze out, resulting
in the correct thermal relic density, but has subdominant contributions to the
DM annihilation today due to velocity suppression. We perform detailed analysis
on the effects of p-wave Sommerfeld enhancement on freeze out. The results show
that with the constraints from the thermal relic density, the required
effective couplings can be reduced by an order of magnitude.
