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A gauged $U(1)_X$ symmetry appended to the Standard Model (SM) isparticularly well-motivated since it can account for the light neutrino massesby the seesaw mechanism, explain the origin of baryon asymmetry of the universevia leptogenesis, and help implement successful cosmological inflation with the$U(1)_X$ breaking Higgs field as the inflaton. In this framework, we propose alight dark matter (DM) scenario in which the $U(1)_X$ gauge boson $Z^\prime$behaves as a DM particle in the universe. We discuss how this scenario with$Z^\prime$ mass of a few keV and a $U(1)_X$ gauge coupling $g_X \simeq10^{-16}$ can nicely fit the excess in the electronic recoil energy spectrumrecently reported by the XENON1T collaboration. In order to reproduce theobserved DM relic density in the presence of such a tiny gauge coupling, wepropose an extension of the model to a two-component DM scenario. The$Z^\prime$ DM density can be comparable to the observed DM density by thefreeze-in mechanism through the coupling of $Z^\prime$ boson to a partnerHiggs-portal scalar DM with a large $U(1)_X$ charge.

Dark Matter $Z^\prime$ and XENON1T Excess from $U(1)_X$ Extended Standard Model