We uncover the general mechanism producing the dark energy .
This is only based on well known quantum physics and cosmology .
We show that the observed dark energy originates from the cosmological quantum vacuum of light particles which provides a continuous energy distribution able to reproduce the data .
Bosons give positive contributions to the dark energy while fermions yield negative contributions .
As usual in field theory , ultraviolet divergences are subtracted from the physical quantities .
The subtractions respect the symmetries of the theory and we normalize the physical quantities to be zero for the Minkowski vacuum .
The resulting finite contributions to the energy density and the pressure from the quantum vacuum grow as \log a ( t ) where a ( t ) is the scale factor , while the particle contributions dilute as 1 / a ^ { 3 } ( t ) , as it must be for massive particles .
The dark energy equation of state P = w ( z ) { \cal H } turns to be w ( z ) < -1 with w ( z ) asymptotically reaching the value -1 from below .
A scalar particle can produce the observed dark energy through its quantum cosmological vacuum provided : ( i ) its mass is of the order of 10 ^ { -3 } eV = 1 meV , ( ii ) it is very weakly coupled and ( iii ) it is stable on the time scale of the age of the universe .
The axion vacuum thus appears as a natural candidate .
The neutrino vacuum ( especially the lightest mass eigenstate ) can give negative contributions to the dark energy .
We find that w ( z = 0 ) is slightly below -1 by an amount ranging from -1.5 \times 10 ^ { -3 } to -8 \times 10 ^ { -3 } while the axion mass results between 4 and 5 meV .
We find that the universe will expand in the future faster than the de Sitter universe , as an exponential in the square of the cosmic time .
Dark energy arises from the quantum vacua of light particles in FRW cosmological space-time in an analogous way to the Casimir effect in Minkowski space-time with non-trivial boundaries .