NASA / ESA / University of California, Santa Cruz / Leiden University / HUDF09 Team

The universe has never made things easy: every time you look away, it becomes bigger, stranger, curiouser. And that’s only the part we can see. As you might have heard if you pay attention to these things (and will be distressed to learn if you don’t), up to 80% of the matter in the universe is simply missing. The Milky Way spins so fast it would fly apart if the gravity of some invisible matter weren’t holding it together. Clusters of galaxies, buzzing around one another like angry bees, would similarly fragment and disperse. And when you run the gravitational numbers, the mysterious matter that keeps all that cosmic disintegration from happening should outweigh the familiar stuff by about 4 to 1.

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It was in the 1930s that astronomer Fritz Zwicky first proclaimed — to general skepticism — that what is now known commonly as dark matter must exist, surrounding most galaxies like a glass paperweight surrounds a butterfly. But not only could physicists not detect the material, they couldn’t even agree on what they should be looking for. Dark matter thus became as much an article of cosmological faith as of well-established theory. Now it appears that faith may have been rewarded. Just as researchers working at Europe’s Large Hadron Collider last year announced that they had bagged the Higgs Boson, so did investigators this week reveal that they’ve found compelling evidence for a type of theorized particle known as a WIMP — for weakly interacting massive particle — and that at least one form of it may be the dark quarry they’ve been hunting for 80 years.

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The new findings come from a team of physicists led by Samuel Ting, of the European Organization for Nuclear Research, relying on data gathered by the Alpha Magnetic Spectrometer (AMS), a detector delivered to the International Space Station in 2011. The purpose of the AMS is to sift incoming cosmic rays — streams of high energy pouring in from outside the solar system — for unusual particles. Dark-matter particles, if they exist, might also get mingled into this cosmic flow, but theories suggest they’d be hard to spot. They can pass through ordinary matter as if it weren’t there (billions could be streaming through your body as you read these words) and they’d be utterly invisible to any sort of telescope.

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If we can’t detect dark matter itself, however, we might detect its byproducts. Theorists think that when two dark-matter particles meet out in space, they’ll occasionally, albeit not often, destroy each other in a tiny burst of energy. That energy would then condense back into entirely different particles: an ordinary electron and its much rarer antimatter counterpart, a positron, which would go speeding away from each other in some random direction.

It’s these positrons that AMS detected — some 400,000 of them in the nearly two years it’s been in operation. The number and energy of the positrons is consistent with what theorists would expect if dark matter really is smashing into itself throughout the Milky Way. So is the fact that the positrons are hitting the detector from all directions, which it should if dark matter truly pervades the Milky Way. Says Jeremiah Ostriker, a Princeton astrophysicist who has been in the forefront of dark-matter theory since the 1970s, “The AMS experiment may — just may — have detected [evidence of] dark-matter decay.”

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Ostriker’s caution, and that of the AMS team at a press conference on Wednesday, is understandable, and not just because the evidence is indirect — like spotting bear tracks instead of the bear. Dark-matter collisions, the scientists acknowledge, are not the only possible source of positrons. They could be streaming off of spinning pulsars, the superdense remnants of exploding stars that also pervade the Milky Way. Their ubiquity would send positrons to us from all directions just the way dark matter would. And even if the pulsars aren’t responsible, says Ostriker, whose new book, Heart of Darkness, chronicles the history of dark-matter research since the very beginning, the positrons “could come from some other strange source” that astronomers haven’t thought of yet.

It’s a good thing, therefore, that the AMS will keep operating for several more years, at least: the more positrons it can find, the firmer the case could become that their source really is dark matter, and physicists might even be able to reason backward and infer the nature of the original particle itself. If AMS really does crack the mystery, Ting, who won a Nobel Prize in 1976 as a co-discoverer of a particle known as the J/psi meson, could well snag another. But he knows better than anyone that it’s a bit too early to start drafting the acceptance speech. The universe may give up its secrets eventually — but it never, ever does so easily.

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