The original experiment was designed to find it as a proof of antimatter, not dark matter.
the AMS is finally delivering on the promise of its original name when "AM" stood for "antimatter."
When Ting sold NASA and DOE on the AMS, he said it might find runaway particles from oases of antimatter, helping solve a deep riddle. The big bang produced matter and antimatter in equal amounts. Soon after, they began colliding and annihilating each other in puffs of gamma rays. But somehow, matter came to dominate the observable universe. That could be because of some fundamental difference between the two—or maybe it was just a coin flip, where certain regions of space came to be ruled by one or the other. Ting's idea to look for those regions galvanized his critics, who considered it outlandish because clumps of antimatter coexisting with normal galaxies would produce more gamma radiation than astronomers observe. Moreover, large antiparticles could not easily survive the journey to the AMS. But if antimatter were there, the AMS would sniff it out—or so the original pitch went.
But each year has also brought one event or so that for all the world looks like it is curving with charge equal to minus two, Ting says—the expected signature of antihelium. The events could just be heliums bouncing unusually off an atom inside the experiment, leading to a misidentification. But the team has used computers to model all the possible paths a particle could take in the detector. "We still do not see any possible way this could come from any background," Ting says. "Many people in the collaboration think we should publish it."
And seem to be holding up for that, leftover antimatter from the big bang.
Why is that? what is the dark matter decay/annihilation that produces that
This is a competing theoretical explanation, based on the theoretical model, example
Galactic Dark Matter (DM) annihilations can produce cosmic-ray anti-nuclei via the nuclear coalescence of the anti-protons and anti-neutrons originated directly from the annihilation process. Since anti-deuterons have been shown to offer a distinctive DM signal, with potentially good prospects of detection in large portions of the DM-particle parameter space, we explore here the production of heavier anti-nuclei, specifically anti-helium
Isn't there any process involving standard model particles that produce anti-helium?
It would be one with very small probabilities because It would have to be a complicated single interaction with 12 quarks and 12 antiquarks, if it were a pair production from a very high energy gamma-field pair production. The models where anti helium coalesces from lower antibaryon number have better probabilities.
Is there any relation with the search for anti-deuteron in dark matter detection experiments?
Anti-deuterons also may appear by coalescence but there are proposals that if dark matter is composed of supersymmetric particles :
measurements of the antiproton cosmic-ray flux at the Earth will be a powerful way to indirectly probe for the existence of supersymmetric relics in the galactic halo. Unfortunately, it is still spoilt by considerable theoretical uncertainties. As shown in this work, searches for low-energy antideuterons appear in the meantime as a plausible alternative, worth being explored. Above a few GeV/n, a dozen spallation antideuterons should be collected by the future AMS experiment on board ISSA. For energies less than about 3 GeV/n, the antideuteron spallation component becomes negligible and may be supplanted by a potential supersymmetric signal. If a few low-energy antideuterons are discovered, this should be seriously taken as a clue for the existence of massive neutralinos in the Milky Way.
It is all very speculative and theoretical.