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An antiproton is relatively easy to form, yet anything heavier, such as antideuterium—an antiproton plus an antineutron—or antihelium—two antiprotons plus typically one or two antineutrons—gets progressively harder to make as it gets more massive.

Another intriguing finding is AMS’s detection of antihelium particles—helium’s antimatter counterparts, which contain two antiprotons and an antineutron—a result the AMS researchers have not yet published.

The AMS, he says, has seen a handful of candidate particles of antihelium-3, made of two antiprotons and an antineutron.

The realm of antimatter is a sort of shadow world in which the particles of our matter-dominated world have mutually annihilating counterparts—the electron has an antimatter partner in the positron, the proton has the antiproton, the neutron has the antineutron, and so forth.

If an antineutron hits a neutron, both turn into energy.