In May 2007, I went to an interesting conference in Canada called Origins of Dark Energy. Dark energy is the part of the universe's content driving it to expand outward at a faster and faster pace--a universal acceleration discovered in 1998. No one knows what causes this effect. Brian Schmidt, one of the co-discoverers of the phenomenon, pointed out the enormously difficulties and costs involved in trying to unravel this mystery. To pin down this effect requires recording the energy released by supernovae billions of years in the past.
The simplest model of dark energy involves adding what is called a "cosmological constant" term to Einstein's equations of general relativity. This is a factor that was proposed by Einstein himself as an attempt to stabilize the universe, but later abandoned by him when he realized that the universe was expanding. Inserting that factor again leads to a kind of cosmological "anti-gravity." It is unclear, however, what physically motivates the inclusion of the term.
Dark energy should not be confused with dark matter. While both are invisible, the former pushes things apart and the latter helps glue them together.