The mystery of dark energy is explained by the hologram cosmological principle trivially and it shows how stupid we all are, me included, that it took this long.
I have generally shown that the Unruh temperature at any horizon is for static LNIF detectors (lowered on a cable, analog to Rindler observers) at distance d from the horizon (e.g. Taylor series in first order for dS metric g00 = 1 - /\r^2 distance d from our retro-causal post-selected future horizon).
kBTHorizon ~ hc/(geometric mean of horizon radius with d)
If the smallest d is the Planck length Lp
kBTHorizon ~ hc/(LpRH)^1/2
The Stephan-Boltzmann Law is that the black body energy density ~ T^4
Voila!
(kBTHorizon)^4 ~ hc/(LpRH)^2
The RHS is the observed dark energy density.