Lambda-CDM model
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 (Lambda, hence Lambda-CDM model). Since energy and mass are related according to the equation Einstein's theory of general relativity predicts that this energy will have a gravitational effect. It is sometimes called a "vacuum energy" because it is the energy density of empty space – the vacuum.