Its simple,
The redshift of light is not related to scale factor like 1+ z = a(1)/a(2) as traditionally thought but as
1+z = [a(1)/a(2)]^2 ........................(1)
This leads to a luminosity distance of c/H*(1+z)(sqrt(1+z) -1)..........(2) , with H half the traditional value, and gives a very good match to supernovae data without dark energy (for small z, d=c/2H*z so matches observations).
The WMAP value for omeg(matter) of 0.25 comes about because omega = rho/rho(crit) and rho(crit) = 3H^2/8*pi*G and the value used for H is twice the real one.
----------------------------------------------------
The reason for (1) is not known for the purposes of this post, but it seems to be the answer to the dark energy problem. The details, including derivation of (2) are in http://vixra.org/abs/1003.0222