[alancalverd]

So it looks as though we have only been exploring half (or less) of the question. If we accept a Big Bang origin of the observable universe, either it was preceded by an enormous amount of energy (and where did that come from, Daddy?) or it produced equal amounts of stuff with positive and negative mass, from nothing. That's a much more attractive idea, intellectually.

This implies that all the particles we have observed to date are better called "reflectons", having all sorts of antisymmetries but positive mass, and there exists an entire universe of true antiparticles with negative mass and whatever other properties we might hypothesise as analogous to charge, spin etc.

[Bored chemist]

matter and antimatter to be mutually repulsive: Just the standard theory

An electron (charge -1) has as its antiparticle: the positron (charge +1). Since opposite charges attract, these will attract each other strongly. That is the standard theory.

However, when you come to uncharged particles (eg anti-neutrons and anti-Hydrogen), the electric field is minimal, and gravitation has a chance to assert itself.

At one time, some cosmologists tried to explain the dominance of matter in our part of the universe by imagining that matter and anti-matter would gravitationally repel each other to opposite ends of the universe.

Most physicists today expect that matter and antimatter will have equal gravitational attraction
- This comes from Einstein's Mass-Energy equivalence.
- A Hydrogen atom has a mass/energy of 939 Mev/c²
- An anti-Hydrogen atom has a mass/energy of 939 Mev/c²
- In Relativity, the gravitational force is between the units of Mass-Energy, and is always attractive
- This is why CERN is creating, storing & experimenting on anti-Hydrogen, to confirm these theories

Quite right. It has been conclusively proved earlier this year.

https://www.space.com/matter-antimatter-same-response-to-gravity

You do realise that means you were proved to be wrong, don't you?