Jerry. Minor nitpick....positrons/electrons are not technically baryonic matter, as they are not baryons. The lightest baryon is the proton. Electrons/positrons are leptons..."light ones" with the muon and tau being the heavier members. Baryons consist of three quarks...leptons have none. Baryons must conserve baryon number in their decays,and have lepton number 0. Leptons conserve lepton number in their decays, have baryon number 0.
Lots of the dark matter popular writers address this lightly. There remains at present no experimental evidence that a normal hydrogen atom has appeared with a newly minted electron and a corroborating proton out of thin air....or contemporary cosmological vacuum. To do so, the Holy Grail of contemporary cosmology, would be the penultimate vindication of Fred Hoyle's original attack on the Big Bang....the generation of matter in a Steady State synthesis, from "empty space", itself.
If that isn't irony, I don't know what is.
pete