Possibly the weirdest idea to be taken seriously by scientists this century, Dark Energy was confirmed recently by an international team of astronomers after completing their analysis of the hubble space telescope Cosmic Evolution Survey (COSMOS). Astronomers over the last decade or so had noticed that the expansion of the universe (predicted by the Big Bang theory, and now routinely observed at observatories around the world) is actually accelerating. Our current knowledge of physics says that the expansion should be slowing because of gravity.
The only way to explain this speeding up is some mysterious new force which affects the universe on a large scale, but not locally. This force is referred to as Dark Energy.