The term "dark energy" was coined by the theoretical cosmologist Michael S. Turner.
Dark Energy's PredecessorBefore physicists knew about dark energy, a cosmological constant, was a feature of Einstein's original general relativity equations that caused the universe to be static. When it was realized the universe was expanding, the assumption was that the cosmological constant had a value of zero ... an assumption that remained dominant among physicists and cosmologists for many years.
Discovery of Dark EnergyIn 1998, two different teams - the Supernova Cosmology Project and the High-z Supernova Search Team - both failed at their goal of measuring the deceleration of the universe's expansion. In fact, they measured not only a deceleration, but a totally unexpected acceleration. (Well, almost totally unexpected: Stephen Weinberg had made such a prediction once)
Further evidence since 1998 has continued to support this finding, that distant regions of the universe are actually speeding up with respect to each other. Instead of a steady expansion, or a slowing expansion, the expansion rate is getting faster, which means that Einstein's original cosmological constant prediction manifests in today's theories in the form of dark energy.
The latest findings indicate that over 70% of the universe is composed of dark energy. In fact, only about 4% is believed to be made up of ordinary, visible matter. Figuring out more details about the physical nature of dark energy is one of the major theoretical and observational goals of modern cosmologists.