Why introduce the mysterious dark energy into the game? Why not just say that the flatness of the universe implies that the dark matter density is close to critical?
There is a considerable amount of evidence that the density of dark matter is very much less than critical. The strongest lines of evidence come from clusters of galaxies:
Tests like those found in measurements of the abundance and baryon fractions of galaxy clusters only probe energy components that can cluster with galaxies. If we want to have a low density of dark matter and still have a flat universe, one has to postulate a new component of energy that is spatially smooth and cannot cluster with galaxies. The simplest example of such a component is Einstein's cosmological constant ? a remarkable type of energy density that is constant in both space and time.
The inference of dark energy taking up some 60-80% of the total energy density is nicely consistent with measurements of the luminosity distance to high redshift supernovae (SNe) which indicate the expansion of the universe is accelerating.
These three tests (CMB for flatness, clusters for the matter density, and SNe for the acceleration) nicely complement each other and currently indicate that dark energy not only exists but is the dominant form of energy density in the universe today. As all of these tests improve in precision, it will be interesting to see whether consistency with a simple cosmological constant for the dark energy remains. Changes in the nature of the dark energy can slightly change the distance CMB photons can travel since recombination and hence change the interpretation of future more precise measurements of the acoustic peak positions.