Hi, My question is: Whats the whole point of solving the Boltzmann equation for relic density of dark matter? I mean, we already know the current density of dark matter in the Universe so that end is fixed by experiment. If we have a model of dark matter evolution then it has to comply to the observed density of dark matter today. Correct? Now given that, do we use the observed density to "fix" the cross-section? Or is it something I am entirely missing?