  We investigate the history of dark energy to explain the present magnitude.
We assume the dark energy is the residual cosmological constant. The most
important channel in the reheating process is the gluon pair productions by QCD
trace anomaly. We argue dark energy decays rapidly by gluon pair emissions
during the reheating and after the big bang. The reheating temperature is
determined by the decay width of dark energy Gamma and the Planck mass M_p as
sqrt{M_P Gamma} ~ 10^6GeV. It is the consequence of Friedmann's equation and an
equilibrium condition Gamma~ H. As the Universe cools below the hadronic scale,
dark energy density is almost frozen. Nevertheless the dark energy further
decreases by emitting two photons. We have estimated the current decay rate of
dark energy from the QED trace anomaly. The consistent solution of Friedmann
equation is in an excellent agreement with the observations. The suppression
factor of dark energy scale is the product of fine structure constant alpha and
curvature perturbation P as 10^{-30}=(\alpha^2P/4\pi)^2. We argue the conformal
symmetry breaking in the both UV and IR are necessary unless dark energy is
subtracted. We also investigated lepto-genesis by adding massive right handed
neutrinos. The realistic lepto-genesis takes place during reheating process.
