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The long-standing Galactic center gamma-ray excess could be explained by GeVdark matter (DM) annihilation, while the DM interpretation seems in tensionwith recent joint limits from different astronomical scale observations, suchas dwarf spheroidal galaxies, the Milky Way halo and galaxy groups/clusters.Motivated by $^8$Be and $^4$He anomalous transitions with possible newinteractions mediated by a vector boson $X$, we consider a small fraction of DMmainly annihilating into a pair of on-shell vector boson $X X$ followed by $X\to e^+ e^-$ in this paper. The Galactic center gamma-ray excess is explainedby this DM cascade annihilation. The gamma rays are mainly from Inverse ComptonScattering emission, and the DM cascade annihilation could be compatible withjoint astrophysical limits and meanwhile be allowed by the AMS-02 positronobservation. The direct detection of this model is also discussed.

Interpretation of the Galactic gamma-ray excess with the dark matter indicated by $^8$Be and $^4$He anomalous transitions