This thread has gotten me thinking (A dangerous pastime, I know), but how do we know that there’s more matter than antimatter?

If antimatter has the same physical properties as matter, then wouldn’t it be indistinguishable from matter? How do we know that some of the galaxies viewed in the Hubble deep field image are not made up entirely of antimatter?

Disclaimer: I’m not some sort of physics quack, it’s just that since this is such a basic supposition, that I’m sure it’s been tested in some way.

The answer is given both in that thread and in the original column.

Nothing exists in splendid isolation in the universe. If there were antimatter galaxies, they would sooner or later interact with normal matter and annihilate each other in spectacular explosions, giving off huge amounts of very visible energy.

We don’t see this in the quantities required, so we’re quite sure that the universe is basically matter throughout.

It’s possible that distant galaxy clusters could be entirely made of antimatter. The distance between an antimatter cluster an a normal matter cluster would be large enough that the tell-tale radiation from particle-antiparticle annihilation would be too small to detect. However, we know of no mechanism that could segregate matter and antimatter on such large scales, but leave all the galaxies within a cluster purely one or the other.

Even in the great voids between the superclusters, you don’t quite have a perfect vacuum. There’s still about one hydrogen atom per cubic meter, or so. So if you have antimatter galaxies, then you must have a transition region where the intergalactic medium is a mixture of matter and antimatter. And even in a medium that diffuse, you’ll still occasionally have gas atoms colliding. The result would be a very distinctive and detectable radiation.