Vacuum energy density
According to quantum theory, empty space is not actually empty but is filled with virtual particles and anti-particles that are created out of nothing and then annihilated. This could, in turn, result in a net density which would behave rather like Einstein’s cosmological constant.
Edward Tryon (City University, New York) suggested in the 1970s that the universe could have appeared out of nothing and contains overall zero energy. This is the vacuum fluctuation permitted by quantum theory.
This vacuum fluctuation energy or zero point energy describes random electromagnetic oscillations that are left in a vacuum after all other energy has been removed. When all matter and energy is removed there is still some energy left. This is explained by the uncertainty principle from quantum mechanics that implies that it is impossible to have a complete absence of energy.