Complex sentence: As photon gas expanded and cooled, some fermions were left over (in extremely small amount ~ 10 -10) because low energy photons could no longer break them apart-and that is the matter we see today in the universe around us.
Split into simpler sentences: As photon gas expanded and cooled, some fermions would be left over (in extremely small amounts ~ 10 -10) because low energy photons could no longer break them apart. Those left-over fermions would have become the matter we see today in the universe around us.