Massive Graviton Dark Matter from a Gapped Continuum

We consider the possibility of dark matter in a warped extra-dimensional theory in presence of a linear dilaton background, with a gapped continuum spectrum, in a brane-world cosmological scenario. Firstly, triggered by self-energy radiative corrections, we study the existence of an isolated resonance of massive gravitons, and its realization as a long-lived feebly interacting dark matter candidate, produced by the freeze-in mechanism. This massive graviton is proved to satisfy all theoretical and experimental constraints, in the sub-MeV mass range. We further consider the close relationship between the existence of this component of dark matter and the presence of an inflaton localized on the brane, with a mass around $10^{11}$ GeV and a sub-TeV reheating temperature, in a brane inflationary scenario that allows to reproduce the most recent cosmological observables. Secondly, the gapped continuum of gravitons, a particular five dimensional realization of the physics of unparticles, is identified as a holographic fluid which can play the role of holographic dark matter. The production of the holographic fluid goes by an ultra-violet freeze-in mechanism, with an abundance mainly depending on the reheating temperature. Depending on the values of the mass gap and the reheating temperature, one or both components of dark matter can be present.