In this chapter I deal with some aspects of the energy density of the Universe. To keep things simple I will ignore the era before electron-positron annihilation, which is dealt with in Chapter 7. Also, I will ignore the neutrino masses because they become important only at a very late stage as described in Section 1.6. With these simplifications the Universe has four components. There are the baryons and the CDM which in the terminology introduced after Eq. (3.3) are matter, and the neutrinos and photons which are radiation.