Spacetime Has a Relaxation Time: A Constitutive Completion of Weak-Field Gravity

Starting from the retarded solution of linearized General Relativity and coarse-graining over galactic scales, a minimal constitutive law emerges — one that requires no new particles, only a finite relaxation time for the weak-field geometric response. The same framework predicts, without fitting to lensing data, an optical projection amplitude A_rel = 19/15 = 1.267, matching the KiDS/GAMA weak-lensing measurement at 0.01σ. Multi-regime tests across galactic rotation curves, halo-mass benchmarks, strong lensing, radio-relic cluster mergers, and peculiar velocity environments all support the same constitutive response. The result is a compact, falsifiable effective theory in which dark-sector phenomenology is delayed weak-field geometry rather than unseen mass.