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The extension of the Standard model by assuming $U(1)_{\rm B-L}$ gaugesymmetry is very well-motivated since it naturally explains the presence ofheavy right-handed neutrinos required to account for the small active neutrinomasses via the seesaw mechanism and thermal leptogenesis. Traditionally, weintroduce three right handed neutrinos to cancel the $[U(1)_{\rm B-L}]^3$anomaly. However, it suffices to introduce two heavy right-handed neutrinos forthese purposes and therefore we can replace one right-handed neutrino by newchiral fermions to cancel the $U(1)_{\rm B-L}$ gauge anomaly. Then, one of thechiral fermions can naturally play a role of a dark matter candidate. In thispaper, we demonstrate how this framework produces a dark matter candidate whichcan address the so-called "core-cusp problem". As one of the small scaleproblems that $\Lambda$CDM paradigm encounters, it may imply an important cluefor a nature of dark matter. One of resolutions among many is hypothesizingthat sub-keV fermion dark matter halos in dwarf spheroidal galaxies are in(quasi) degenerate configuration. We show how the degenerate sub-keV fermiondark matter candidate can be non-thermally originated in our model and thus canbe consistent with Lyman-$\alpha$ forest observation. Thereby, the smallneutrino mass, baryon asymmetry, and the sub-keV dark matter becomeconsequences of the broken B-L gauge symmetry.

Degenerate Fermion Dark Matter from a Broken $U(1)_{\rm B-L}$ Gauge Symmetry