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