dark matter interacts with ordinary matter not just grav- itationally. While the WIMP miracle suggests that dark matter is charged under the Standard Model electroweak force, we will see that these observations provide impres- sive evidence for dark matter interacting with ordinary matter through a new force, mediated by a new 50 MeV – 1 GeV mass gauge boson . In addition to explaining any or all of these observations, dark matter charged un- der this new force automatically has the correct thermal relic abundance observed today by virtue of its interac- tions via the new force carrier, reproducing the success of the WIMP dark matter hypothesis. 

The satellites PAMELA [50] and Fermi [51], the balloon-borne detector ATIC [52], the ground-based tele- scope HESS [53, 54], as well as other experiments, ob- serve an excess in the cosmic-ray flux of electrons and/or positrons above backgrounds expected from normal as- trophysical processes. If their source is dark matter anni- hilation or decay, synchrotron radiation from these elec- trons and positrons could also explain the “WMAP haze” near the Galactic center [55], which consists of an ex- cess seen in the WMAP Cosmic Microwave Background data. In addition, starlight near the Galactic center would inverse Compton scatter off the high energy elec- trons and positrons and produce an excess in gamma- rays. A detection of a gamma-ray excess towards the Galactic center region in the gamma-ray data obtained with the Fermi satellite was recently reported in [56], and has been dubbed the “Fermi haze”. 

Taken together, these observations by several experi- mental collaborations provide compelling evidence that there is an unexplained excess in cosmic-ray electrons and positrons in our Galaxy. Given the firm evidence for a 22% dark matter content of the Universe, a very natural source of these excesses is dark matter annihila- tion. However, two features of these observations are in- compatible with annihilation of ordinary thermal WIMP dark matter. They instead provide impressive evidence that dark matter is charged under a new U (1) ′ and anni- hilating into the A ′ , which decays directly into electrons and positrons, or into muons that decay into electrons and positrons, see Figure 2 (left) (see e.g. [5, 57–63]). These two features are: 

• The annihilation cross-section required to explain the signal is 50-1000 times larger than the ther- mal freeze-out cross-section for an ordinary WIMP that is needed to reproduce the observed dark mat- ter relic density. This can be explained if dark matter interacts with a new long range force me- diated by an O (GeV) mass gauge boson, which allows the dark matter annihilation cross-section ( 〈 σv 〉 ) to be enhanced at low dark matter velocities, i.e. 〈 σv 〉 ∝ 1 /v . In this case, in the early Universe when the dark matter velocity was high ( ∼ 0 . 3 c ), the annihilation cross-section that determines the relic abundance can naturally be the same as that of an ordinary WIMP and reproduce the WIMP miracle. However, in the Milky Way halo now, the 

dark matter has a much lower velocity ( v ∼ 10 − $^{3}$c ), leading to a large increase in the annihilation cross- section that is required to explain the cosmic-ray data. The enhancement at low velocities through a new long-range force is very well known and called the Sommerfeld effect [64]. 

• The PAMELA satellite did not see an anti-proton excess [65], which strongly suggests that dark mat- ter annihilation is dominantly producing leptons, and not baryons. If dark matter is interacting via a O ( GeV ) mass force particle in order to have a large annihilation rate via the Sommerfeld mechanism, then annihilations into the force carrier automat- ically fail to produce any baryons. Kinematically, the force carriers cannot decay into baryons, and are instead forced to decay into the lighter charged leptons. Thus, annihilation products of dark mat- ter are leptonic in this case. 

To explain the additional sources of evidence for a new GeV scale force, we briefly summarize the consequence for dark matter mass spectra that follow from dark mat- ter carrying a charge under a new force. If dark mat- ter is charged under a non-Abelian force that acquires mass, then radiative effects can split all components of the dark matter with size, δ ∼ α$_{D}$ Δ m$_{W}$$_{D}$ , where α$_{D}$ is the non-Abelian fine structure constant and Δ m$_{W}$$_{D}$ is the splitting of gauge boson masses [57]. Typically, these splittings are Δ m$_{W}$$_{D}$ ∼ α$_{D}$m$_{W}$$_{D}$ ∼ 1 − 10 MeV for m$_{W}$$_{D}$ ∼ 1 GeV [57]. Thus, δ ∼ 100 keV for α$_{D}$ ∼ 10 − $^{2}$. These splittings are completely analogous to the split- tings that arise between the π ± and π 0 from Standard Model SU(2) breaking. If instead a non-Abelian force confines at a scale Λ$_{D}$ ∼ GeV, then a heavy-flavor meson can be cosmologically long-lived and thus a dark matter candidate [66]. Hyperfine interactions can naturally in- duce ∼ 100 keV splittings of the dark matter particles in this case. We emphasize that the GeV scale force carrier particles mediate quantum corrections that generate the 100 keV and 1-10 MeV splittings of dark matter states [57, 66–68]. 

When mass splittings arise, A ′ mediated interactions of dark matter with ordinary matter as well as dark matter self-interactions are dominated by inelastic col- lisions [57]. The direct dark matter detection experi- ment DAMA/LIBRA as well as the INTEGRAL tele- scope provide intriguing evidence for such interactions. The DAMA/NaI [69] and DAMA/LIBRA [70] experi- ments have reported an annual modulation signal over nearly eleven years of operation with more than 8 σ sig- nificance. Modulation is expected because the Earth’s velocity with respect to the dark matter halo varies as the Earth moves around the sun, and the phase of the observed modulation is consistent with this origin. A simple hypothesis that explains the spectrum and mag- nitude of the signal, and reconciles it with the null results of other experiments, is that dark matter-nucleus scatter- 

