A symmetry trick makes freeze-in dark matter naturally tuning-free

Freeze-in dark matter production demands couplings so small they feel unnatural — roughly one part in ten billion. By placing a sterile singlet inside a 3-3-1 electroweak extension and enforcing a Z₁₃ discrete symmetry, the authors force all low-dimensional portals to close, leaving only a dimension-six operator to connect dark matter to the Standard Model. The required tiny coupling then emerges automatically from the ratio of the electroweak scale to a high Peccei-Quinn scale. Boltzmann equation solutions show the mechanism reproduces the observed relic density at keV masses, satisfying Lyman-alpha constraints while addressing the cusp-core and missing satellites tensions in small-scale structure.