Dark energy, which forms almost three-quarters of the universe, is the most mysterious stuff known to man. A new set of Chandra X-Ray Observatory data, however, has given scientists good information on what dark energy might actually be.
Turns out, it looks suspiciously like Einstein's cosmological constant, a factor he added to his theory of general relativity. He once considered it his greatest blunder.
Dark Energy Could Be Einstein's Cosmological Constant | Wired Science from Wired.com