> The current best guess is that there is some matter that we can't see for some reason. Nobody is sure what it is.

My pet conjecture (it's not detailed enough to be a hypothesis) is that this is related to the baryon asymmetry problem.

The antimatter symmetry problem is more than just baryons, despite the name, as we also have more electrons than positrons, not just more protons/neutrons than anti-protons/anti-neutrons.

There's a few possibilities:

1) the initial value just wasn't zero (an idea I heard from Sabine Hossenfelder)

2) the baryon number is violated in a process that requires conservation of charge

This would suggest antiprotons or antineutrons do something which involves the positron at the same time, so perhaps the anti-neutron is weirdly stable or something — neutron decay is a weak force process, and that can slightly violate the charge conjugation parity symmetry, so this isn't a completely arbitrary conjecture.

If we've got lots of (for example) surprise-stable anti-neutrons all over the place… it's probably not a perfect solution to the missing mass, but it's the right kind of magnitude to be something interesting to look at more closely.

3) the baryon number (proton/neutron/etc.) and/or lepton number (electron/positron/muon/etc.) is violated in a process that does not require conservation of charge.

If you have some combination of processes which don't each conserve charge, you're likely to get some net charge to the universe (unless the antiproton process just happens to occur at the same rate as the positron process); in quantum mechanics I understand such a thing is genuinely meaningless, while in GR this would contribute to the stress energy tensor in a way that looks kinda like dark energy.

But like I said, conjecture. I'm not skilled enough to turn this into a measurable hypothesis.