A very light boson of mass $\mathcal{O}({10}^{-22})\mathrm{eV}$ may potentially be a viable dark matter (DM) candidate, which can avoid phenomenological problems associated with cold DM. Such “fuzzy DM (FDM)” may naturally be an axion with a decay constant $f_a\sim 10^{16}–10^{18}\mathrm{GeV}$ and a mass $m_a\sim {\mu }^2/f_a$ with $\mu \sim 10^2\mathrm{eV}$. Here, we propose a concrete model, where $\mu$ arises as a dynamical scale from infrared confining dynamics, analogous to QCD. Our model is an alternative to the usual approach of generating $\mu$ through string theoretic instanton effects. We outline the features of this scenario that result from various cosmological constraints. We find that those constraints are suggestive of a period of mild of inflation, perhaps from a strong first order phase transition, that reheats the standard model (SM) sector only. A typical prediction of our scenario, broadly speaking, is a larger effective number of neutrinos compared to the SM value $N_{\text{eff}}\approx 3$, as inferred from precision measurements of the cosmic microwave background. Some of the new degrees of freedom may be identified as “sterile neutrinos,” which may be required to explain certain neutrino oscillation anomalies. Hence, aspects of our scenario could be testable in terrestrial experiments, which is a novelty of our FDM model.

• Received 13 January 2017

DOI:https://doi.org/10.1103/PhysRevLett.118.141801