With stronger magnets, larger collecting areas, longer flights, and improved electronics, these experiments had steadily increasing sensitivity and detected more and more antiprotons, but none detected antihelium.
Therefore, no more than one antihelium per 1.5 million cosmic rays of normal helium could be reaching Earth, concluded a 2002 analysis by Makoto Sasaki of the BESS team.
They calculated its sensitivity at one antihelium among roughly 100 million nuclei of normal helium.
If it ever flies, AMS-02 will improve the sensitivity of the search to about one antihelium in a billion helium nuclei.
An antihelium nucleus found in Earth's vicinity would have had a long and arduous trip.
If it did traverse intergalactic space, the antihelium nucleus would eventually cross a threshold between the antimatter region and the neighboring matter region.
Finally, if an antihelium nucleus is to be recorded, it must approach the neighborhood of the Milky Way.
Even with the enormous number of particles that would be flowing out of an antimatter galaxy, the odds of an antihelium nucleus reaching a detector near Earth are small.
"I don't see any reason why, in our conventional understanding of cosmology, we would see any antihelium in the universe," says Sean Carroll of the California Institute of Technology in Pasadena.
Some scientists have suggested that any region of antimatter might be so large that antihelium particles would never make it to Earth.