In the mean time, it appears that the existence of Dark Energy has now an additional independent evidence.
That period was in fact a crucial moment in the tug of war between the outward push of dark energy and the inward pull of gravity. In other words, the universe had stretched so much by this point that its dark energy made it difficult for galaxy clusters to pull matter in from far away. Not only did existing galaxy clusters slow their growth as a result, the rate at which new clusters were formed also declined.
"What we saw was an unmistakable effect of dark energy," says Vikhlinin, whose team is publishing the results in two papers in the 10 February 2009 issue of The Astrophysical Journal. The timing of dark energy's dampening effect on cluster growth coincides with findings by supernovae researchers showing that the universe's expansion had been decelerating before beginning to accelerate 5.5 billion years ago.