  We show that the 511 keV gamma ray excess observed by INTEGRAL/SPI can be
more robustly explained by exciting dark matter (DM) at the center of the
galaxy, if there is a peculiar spectrum of DM states chi_0, chi_1 and chi_2,
with masses M_0 ~ 500 GeV, M_1 <~ M_0 + 2 m_e, and M_2 = M_1 + delta M >~ M_0 +
2 m_e. The small mass splitting delta M should be <~ 100 keV. In addition, we
require at least two new gauge bosons (preferably three), with masses ~100 MeV.
With this spectrum, chi_1 is stable, but can be excited to chi_2 by
low-velocity DM scatterings near the galactic center, which are
Sommerfeld-enhanced by two of the 100 MeV gauge boson exchanges. The excited
state chi_2 decays to chi_0 and nonrelativistic e+e-, mediated by the third
gauge boson, which mixes with the photon and Z. Although such a small 100 keV
splitting has been independently proposed for explaining the DAMA annual
modulation through the inelastic DM mechanism, the need for stability of chi_1
(and hence seqestering it from the Standard Model) implies that our scenario
cannot account for the DAMA signal. It can however address the PAMELA/ATIC
positron excess via DM annihilation in the galaxy, and it offers the
possibility of a sharper feature in the ATIC spectrum relative to previously
proposed models. The data are consistent with three new gauge bosons, whose
couplings fit naturally into a broken SU(2) gauge theory where the DM is a
triplet of the SU(2). We propose a simple model in which the SU(2) is broken by
new Higgs triplet and 5-plet VEV's, giving rise to the right spectrum of DM,
and mixing of one of the new gauge bosons with the photon and Z boson. A
coupling of the DM to a heavy Z' may also be necessary to get the right relic
density and PAMELA/ATIC signals.
