Chandra X-ray observatory has given new evidence of the existence of dark energy. This striking result has been obtained with the observation of Abell 85. This is a galaxy cluster located about 740 million light years from Earth.
I have found another beautiful article on NewYork Times about (see here). Dark energy is currently described by a non-null cosmological constant in the Einstein equations but no explanation is known whatsoever about its origin. Currently, a famous explanation uses the anthropic principle but was criticized by Steven Weinberg (see here). This argument seems to support very strongly the landscape view of string theory but whatever relies on anthropic arguments cannot be said to be really satisfactory for our understanding. So, physicists are looking for something better but nothing is in view. Rather, quantum field theory gives a too large account for this constant. As this is a major breakthrough in our understanding of universe, it is a serious reason to stay tuned waiting for more information.