The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space. This is the cosmological constant, usually represented by the Greek letter Λ (Lambda, hence the name Lambda-CDM model). Since energy and mass are related according to the equation E = mc, Einstein's theory of general relativity predicts that this energy will have a gravitational effect. It is sometimes called vacuum energy because it is the energy density of empty space – of vacuum.