  The classical Friedman equations of time-varying Hubble function $H$,
dark-energy and matter densities couple to quantised field equations for
massive modes $M\gg H$. Numerically solving these equations, we show the
particle-antiparticle pairs production and oscillation in microscopic time
scale ${\mathcal O}(M^{-1})$. A massive pair plasma state is formed in
macroscopic time scale ${\mathcal O}(H^{-1})$. Its density and pressure
introduce the interaction of matter and dark energy densities in the Friedman
equations. Focusing on epochs after reheating, we show that the negative dark
energy tracks down the radiation energy in the radiation epoch. Such tracking
dynamics ends, and dark energy becomes positive in the matter epoch. The matter
converts to dark energy, and their present values are comparable, explaining
the cosmic coincidence. As a result, a class of effective interacting dark
energy models is advocated to confront cosmological observations.
