Dark Matter Halo Anomalies as Signatures of Late-Time Structural Over-Constraint

Small-scale discrepancies between observations of galaxy halos and predictions from collisionless cold dark matter (CDM) simulations—including the cusp–core problem, the too-big-to-fail phenomenon, and the diversity of rotation curves—are commonly interpreted as evidence for new dark matter microphysics. In response, self-interacting dark matter (SIDM) models have demonstrated significant phenomenological success, reproducing many observed halo properties through enhanced internal energy redistribution. However, the physical origin and environmental variability of the required interaction cross-sections remain unresolved. In this paper, we propose an alternative interpretation: these anomalies arise from late-time structural over-constraint, rather than from missing forces or particles. We frame dark matter halos as systems evolving within regime-dependent constraint landscapes, where early permissive evolution gives way to structural saturation and damping. In such over-constrained regimes, collisionless dynamics are insufficient to describe internal reconfiguration, leading to arrested relaxation, hysteresis, and diversity of halo outcomes. Within this framework, SIDM operates as an effective mediation model, restoring degrees of freedom that are suppressed in saturated systems without necessarily corresponding to a fundamental interaction. Gravothermal core formation, recontraction, and collapse are reinterpreted as terminal adjustments within narrowed admissible configuration spaces, not as evidence of new dark sector physics. By treating halo-scale discrepancies as diagnostic markers of regime misclassification, this work reconciles the empirical success of SIDM with the absence of a compelling microphysical explanation. We argue that dark matter halo anomalies reflect the structural consequences of late-time constraint accumulation, completing the evolutionary arc from early permissiveness to saturated cosmic environments.