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 $M_0\sim 500\mathrm{GeV}$, $M_1\lesssim M_0+2m_e$, and $M_2=M_1+\delta M\gtrsim M_0+2m_e$. The small mass splitting $\delta M$ should be $\lesssim 100\mathrm{keV}$. In addition, we require at least two new gauge bosons (preferably three), with masses $\sim 100\mathrm{MeV}$. 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 sequestering 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 vacuum expectation values, 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.

• Received 4 February 2009

DOI:https://doi.org/10.1103/PhysRevD.79.063530