Cosmologists from Princeton are working on new tests that could help to explain the nature of "dark energy", a mysterious force accelerating the expansion of the Universe. It could be an unknown form of energy, or it could be that Einstein's General Theory of Relativity breaks down at very large scales. The researchers will track the rate at which galaxy clusters have grown in time. If this growth is consistent, it'll mean that dark energy is at work; otherwise, it could mean problems with Einstein's predictions.