DARK ENERGY.It is called Dark Energy and is different than Dark Matter (which is around galaxies). Dark energy is mostly described by a non-zero cosmological constant. This takes some history.When Einstein came up with his equations for gravity, he realized that they did not allow a static universe. This was a problem for him philosophically, so he added another term to his basic equations. This term had a constant part which is called the cosmological constant. Einstein showed that a particular value for this constant allowed a static universe. When the expansion of the universe was discovered by Hubble, Einstein called the constant 'his greatest mistake'. For decades after this, Einstein's original equations were used instead of the modified ones with the constant.Now fast forward to a couple of years ago. It was found that the expansion rate is actually accelerating. This can be explained by having a non-zero cosmological constant.
