Dark Energy

Until the late 1990s, astronomers expected the expansion of the universe to be slowing down, because gravity between all the matter in the universe pulls it back.In 1998, two teams measured the distances to very distant galaxies using type 1a supernovae. These are exploding white dwarfs that all reach almost the same peak luminosity, so they act as 'standard candles': measuring how bright one appears from the Earth gives its distance. Comparing these distances with the galaxies' red shifts showed that the distant supernovae were fainter, and so further away, than expected. The expansion of the universe is accelerating.Something must be pushing the universe apart against gravity. This unknown form of energy is called dark energy. It is thought to be spread evenly through all of space and to act on the largest scales. Unlike matter, its density does not fall as space expands, so as the universe grows, its effect becomes more important compared with gravity.What dark energy actually is remains one of the biggest open questions in physics. One idea is that it is an energy of empty space itself.Current measurements suggest the mass-energy of the universe is made up of roughly:dark energy ≈ 68%    dark matter ≈ 27%    ordinary matter ≈ 5%So everything we can see (stars, gas, planets and ourselves) is only about one twentieth of the contents of the universe. Dark energy also affects the fate of the universe: if the acceleration continues, galaxies will move apart ever faster and the universe will expand forever.