[AnotherGoodName]

They do. In fact i always thought this was the answer to the matter/antimatter imbalance.

Feynmann diagrams literally show anti matter as the same particle as a matter particle, just travelling back in time (see election/positron interactions).

So what happens when matter and antimatter are created in a big boom? Well the antimatter is in the past, we're here in the future.

[xenadu02]

Not sure how we could ever prove something like that but it's certainly an amusing and symmetrical view of the universe: big bangs create two universes, moving in opposite time directions from each other. Each seeing the other's particles as anti-particles.

Thinking about it though: photos are their own antiparticle. So I'd expect to see a lot more cosmic microwave background than we should because at least in the early days the antimatter universe would have been visible to us?

Side note: how can photos be their own antiparticle? Same reason they move at the speed of causality. They have no mass thus do not experience the flow of time themselves. So they do not annihilate with themselves. From a photon's POV a trip across the universe is instant.

[AnotherGoodName]

I'm not at all invested in the above except as a vague thought experiment but imho...

No theory can match CMB as well as the current theories that add cosmic inflation on an as needed basis in time and space to make CMB observations match perfectly. But cosmic inflation is a completely unknown mechanism (no explanation at the level of particle physics) and it was added specifically to make CMB match perfectly. In some ways i think the current 'we observe CMB to be this so we'll move these free variables to make it match' make CMB non-falsifiable. You could add similar free variables to any theory to make CMB match.

[wizzwizz4]

In the early days, the universe was opaque.