This is a very interesting topic which I have engaged in at various times:
The Law of Conservation of Entity Count
Electrons From Photons
Especially in the thread entitled:
The Law of Conservation of Entity Count; I learned that it is very likely that photons are not the basic building block of the universe.
I would like, very much, for that to be the case because it fits my understanding of the universe better but particle physics seems to say otherwise.
If photons are not the elemental entity of the universe then the big bang was not an expanding ball of photons as
astromark and
Professor Mayhem suggest but instead the big bang was composed of a multitude of different entities: photons, quarks, neutrinos, and who knows what else.
The big bang was a true soup with a variety of ingredients without any of which would result in an unpalatable slop.
The universe was at its simplest state at the big bang but it was definitely not a simple single-entity beginning: photons.