  In the presence of the gravitational field, the energy density of matter no
longer coincides with its mass density. A discrepancy exists, of course, also
between the associated power spectra. Within the $\Lambda$CDM model, we derive
a formula that relates the power spectrum of the energy density to that of the
mass density and test it with the help of N-body simulations run in comoving
boxes of 2.816 Gpc/$h$. The results confirm the validity of the derived formula
and simultaneously show that the power spectra diverge significantly from one
another at large cosmological scales.
