WMAP determined that the universe is flat, from which it follows that the mean energy density in the universe is equal to the critical density (within a 0.5% margin of error). This is equivalent to a mass density of 9.9 x 10-30 g/cm3, which is equivalent to only 5.9 protons per cubic meter.
There seem to be two parts to your question
- Why is the universe flat?
- Why is the energy density of the dark energy component so similar to the matter density?
The theory of inflation was proposed to explain 1) as well as the 'horizon problem'. For 2) there doesn't seem to be a generally accepted and convincing answer at the moment.
Supporters of the mainstream 'Concordance Cosmology' believe that answers to your questions have been found (for 1) or will be found for 2).
Others believe that a more radical approach is needed.