A sign-switching, positivity-preserving energy exchange between dark matter and dark energy: theory, tests and a falsifiable S8 prediction

We propose an energy exchange between dark matter and dark energy whose direction reverses when the two densities cross, Q = 3δHρ_dm ρ_de(ρ_de − ρ_dm)/(ρ_dm + ρ_de)². It is the lowest-order kernel that keeps both densities positive and is antisymmetric under exchange of the two components. We give a Lagrangian realisation that has no fifth force, derive the stability conditions, and implement the model in a modified CLASS code. We test it against Planck 2018 CMB, DESI DR2 BAO, four supernova catalogues, growth-rate data and weak lensing. With the full dataset including weak lensing, the model improves on ΛCDM by Δχ² ≈ −13 (DES supernovae) and −8 (Pantheon+), outperforming the CPL parametrisation. It predicts S8 ≈ 0.79, a non-phantom w ≈ −0.88 and a sign flip at z ≈ 0.55. Euclid's first data release can test this at about 4σ.