Horizon looks at dark energy - the mysterious force that is unexpectedly causing the universes expansion to speed up. The effects of dark energy were discovered in 1998 but physicists still dont know what it is. Worse, its very existence calls into question Albert Einsteins general theory of relativity - the cornerstone of modern physics. The hunt for the identity of dark energy is on. Experiments on earth and in space generate data that might provide a clue, but there are also hopes that another Einstein might emerge - someone who can write a new theory explaining the mystery of the dark energy.