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 NI with masses M-N;I, and a singlet scalar R that mixes with the Standard Model Higgs doublet Phi by an angle alpha. A symmetry forbids the 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)proportional to Sigma(I)m nu;M-I(N;I)2=(v(h)m(2)H), 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) 5 & times; 10(-11)(M-N=10 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) << 1 and m chi > m(Hp,) the relic density is set by p-wave annihilation chi chi & strns;-> HpHp (with H(p )the Higgs-like particle of the dark sector), and the dark-sector Yukawa couplings required to reproduce the observed abundance are O(0.1-1), as in the standard WIMP case. For heavy-neutrino masses greater than or similar to 10 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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