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 } \sim 500 GeV , M _ { 1 } \mbox { \raisebox { -2.58 pt } { ~ { } $ \stackrel { < } { \sim } $~ { } } } M _ { 0 } +2 m _ { e } , and M _ { 2 } = M _ { 1 } + \delta M \mbox { \raisebox { -2.58 pt } { ~ { } $ \stackrel { > } { \sim } $~ { } } } M _ { 0 } % +2 m _ { e } .
The small mass splitting \delta M should be \mbox { \raisebox { -2.58 pt } { ~ { } $ \stackrel { < } { \sim } $~ { } } } 100 keV .
In addition , we require at least two new gauge bosons ( preferably three ) , with masses \sim 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 can not 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 ^ { \prime } may also be necessary to get the right relic density and PAMELA/ATIC signals .