Several articles I've read say that the basis for BBN is that we know how particles react here on earth, so we can extrapolate that and make precise predictions for the composition of the universe at a given energy level. For example, at 100 Billion K, we have a pretty good idea how neutrons, protons, electrons, photons and the like would combine, recombine and form an equilibrium based on a statistical model. So here's the part I don't get: Dark Matter has the same electrical properties as neutrons. Dark matter participates in Weak Nuclear reactions. So how come none of these reactions involve Dark Matter (which was abundant as a primordial particle at this time)?