Prior to the late 1920s the view of the universe was that it was static, or stars we could see seemed to be in fixed relative positions. As Albert Einstein developed his General Theory of Relativity around 1915, which is still our best description of gravity and the development of the universe, he accounted for the static state of the universe with a term in his field equations called the cosmological constant. Essentially, this was equivalent to a type of anti-gravity force that balanced the mutual gravitational attraction between objects in the universe and held everything fixed. However, the observation by Edwin Hubble that galaxies were rushing away from each other led to Big Bang models of the universe and required Einstein to remove his cosmological constant, leading to his famous quote that the constant was his "biggest blunder."
Several years ago, two independent observations of distant supernovae showed that some objects seem to be speeding up as they rush away from us. In other words, it seems as if there is a repulsive force stronger than the gravitational attraction on these objects, acting like an 'anti-gravity' force should. The term 'dark energy' has entered the scientific literature, almost sounding like it was stolen from Star Wars.
Since those preliminary data were published, more groups of researchers have looked at many more supernovae, and the results appear to resemble Einstein's original cosmological constant. Einstein asumed a value of -1.0, and data place the value at something less than -0.85. This acceleration appears to be a real phenomenon, and it does not appear to vary with distance. These new data will help reduce the number of theoretical models attempting to describe what dark energy is and how it behaves, and it shows that Einstein may have been on the right track after all...why does that not seem all so surprising?!