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Lastly, we say a few words about the nature of the dark matter and the dark energy. From calculations and observations, the dark matter must be in the form of CDM , C old D ark M atter. A CDM particle is cold in that it has negligible (meaning non-relativistic) random motion. It has negligible inter- action with other particles and also negligible self-interaction, hence must be non-baryonic, with its only real presence being observed via its gravitational effect on galactic dynamics. Regarding dark energy, the simplest realization is a Cosmological Constant , denoted by Λ. This is a spatially and tem- porally constant term that is added to Einstein’s Field Equation, having the effect of negative pressure. 

Taking all of our discussion so far into account, we are presented with the ΛCDM model as our Concordance Model of Cosmology, which con- tains just six independent parameters: H 0 , Ω B , Ω CDM , τ , P$_{ζ}$ ( k 0 ) and n$_{s}$ . Fig. 2.3 shows a plot of the agreement between the ΛCDM model and the data obtained from the Planck spacecraft. 

