© NASA, CXC, MSFC, M. Bonamente et al.
The six galaxy clusters shown here were observed with the Chandra X-ray Observatory. It is tricky to detect galaxy clusters in visible-light images because galaxies in the cluster are difficult to distinguish from galaxies in front of and behind the cluster. It is much easier to detect galaxy clusters in x-ray images because there is very hot gas between the galaxies of a cluster that emits x-rays. Chandra's complete set of galaxy cluster observations show that dark energy stifled the growth of these massive structures over the past seven billion years. (Unit: 11)