Tentative evidence for excess GeV-scale gamma rays from the galactic center has been corroborated by several groups , including the Fermi collaboration , on whose data the observation is based .
Dark matter annihilation into standard model particles has been shown to give a good fit to the signal for a variety of final state particles , but generic models are inconsistent with constraints from direct detection .
Models where the dark matter annihilates to mediators that subsequently decay are less constrained .
We perform global fits of such models to recent data , allowing branching fractions to all possible fermionic final states to vary .
The best fit models , including constraints from the AMS-02 experiment ( and also antiproton ratio ) , require branching primarily to muons , with a \sim 10 - 20 \% admixture of b quarks , and no other species .
This suggests models in which there are two scalar mediators that mix with the Higgs , and have masses consistent with such a decay pattern .
The scalar that decays to \mu ^ { + } \mu ^ { - } must therefore be lighter than 2 m _ { \tau } \cong 3.6 GeV .
Such a small mass can cause Sommerfeld enhancement , which is useful to explain why the best-fit annihilation cross section is larger than the value needed for a thermal relic density .
For light mediator masses ( 0.2 - 2 ) GeV , it can also naturally lead to elastic DM self-interactions at the right level for addressing discrepancies in small structure formation as predicted by collisionless cold dark matter .