I've been reading about the early universe (The book: The First Three Minutes) and it prompted a few questions: Is it right that a dominant part of the early universe was electrons and positrons? Given that electrons and positrons were a big part of the early universe, was there some early process whereby they ended up becoming masses (or just the positrons did)? I'm trying to understand how the balance in numbers of electrons, protons and neutrons came about. I find it difficult to get my head around a particle having no mass (photons). Is it that their mass is so insignificantly small, compared to protons and neutrons, that their mass has no measurable effect? Or is it that they actually do have zero mass? Edited to correct something I misread. Apologies.