Second, Einstein’s E = mc^2 is not an equivalence of energy and matter. It is the equivalence of energy and mass (i.e. the number of kilograms). So a unit volume of the empty space carries some mass equivalent to the energy – it’s a mass of a few protons per cubic meter. But the E = mc^2 equation does not imply, in any sense, that the mass equivalent to the energy has to take the form of localized particles. It may be dispersed, much like the cosmological constant – whose generalized form is also called dark energy. The main reason why the vacuum contains mass is that this mass contributes to the curvature of spacetime – the gravitational field of mass – and be sure that dark energy does. That is why it was introduced.
Dark energy, unlike mass, carries a negative pressure, and it’s the real source of the accelerating expansion it induces. Ordinary matter has attractive gravity.
— answered May 3, 2011 at 4:33
— Lubos Motl
2012.10.09 Tuesday ACHK