We present a secluded dark-matter (DM) framework based on an extra $U(1{)}_X$ gauge symmetry. The model contains a Dirac DM particle $\chi$, three heavy neutrinos $N_I$ with masses $M_{N,I}$, and a singlet scalar $R$ that mixes with the Standard Model Higgs doublet $\mathrm{\Phi }$ by an angle $\alpha$. A symmetry forbids the $\mathrm{\Phi }-R$ portal at tree level; the leading portal then arises at one loop from the same Yukawa structures that generate active neutrino masses $m_{\nu ,I}$, implying $\tan (2\alpha )\propto {\sum }_I{m}_{\nu ,I}{M}_{N,I}^2/(v_h{m}_H^2)$, where $v_h$ and $m_H$ are the Standard Model Higgs vacuum-expectation-value and mass. For heavy-neutrino masses in the multi-TeV range, this yields a naturally tiny mixing, $\tan (2\alpha )\sim 5\times {10}^{-11}{(M_N/10\mathrm{TeV})}^2$, which strongly suppresses DM signals in direct, indirect, and collider searches. For PeV-scale heavy neutrinos the loop-induced portal is enhanced and the DM-nucleon cross section can instead enter the reach of direct-detection experiments. The visible and dark sectors thermalize at temperatures of order a few times the mass of the lightest heavy neutrino, then subsequently decouple, and typically evolve with a slightly hotter dark bath. In the secluded regime, with $\tan (2\alpha )\ll 1$ and $m_{\chi }>m_{H_p}$, the relic density is set by $p$-wave annihilation $\chi \overline{\chi }\to H_p{H}_p$ (with $H_p$ the Higgs-like particle of the dark sector), and the dark-sector Yukawa couplings required to reproduce the observed abundance are $\mathcal{O}(0.1-1)$, as in the standard WIMP case. For heavy-neutrino masses $\gtrsim 10\mathrm{TeV}$, the mediator decays before nucleosynthesis without spoiling big bang nucleosynthesis observables, while the tiny portal suppresses present-day signals below current and near-future sensitivities. This links two long-standing puzzles—the absence of DM signals and the smallness of neutrino masses—within a predictive thermal framework.

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• Dark matter
• Extensions of gauge sector
• Particle dark matter
• Neutrinos