Spacetime Geometric Memory as an Effective Dark Halo

What if the gravitational field does not adjust instantaneously to matter, but carries a finite memory of its past? We test this idea with a constitutive framework where weak-field spacetime relaxes over a local clock, τ. Calibrated exclusively with SPARC rotation curves (τ≃36.6 Myr) and held fixed thereafter, this single clock predicts the gas-lensing offset in cluster mergers (r=0.925), reproduces the isolated GAMA lensing profile (χ2/N=1.22), and recovers the environmental dependence of peculiar velocities (ΔAIC=54.3). When carried into the native Planck 2018 TT/TE/EE+lensing likelihood, the externally calibrated clock yields a fit indistinguishable from ΛCDM (Δχ2=−0.046). A joint analysis with BAO, Pantheon+, and growth improves over ΛCDM by Δχ2=−11.92 (ΔAIC=−7.92). Weak lensing independently measures the optical response of the residual geometry at 8.5σ, separating the relaxation clock from the lensing projection. The evidence suggests a structural degeneracy: part of the effective halo may be delayed curvature rather than unseen particulate mass alone.