Dark matter has teased and obsessed astronomers since the 1930s, when the Caltech astronomer Fritz Zwicky deduced that some invisible “missing mass” was required to supply the gravitational glue to hold clusters of galaxies together.

According to recent measurements by the Planck spacecraft, about 27 percent of the universe, by mass, is composed of some unknown form of matter unlike the atoms that make up us and everything we can see. Astronomers cannot see it, but they can detect its gravitational tug pulling the galaxies and stars around.

Figuring out what this stuff is is important for more than cosmology. The most favored candidates for its identity are as yet undiscovered particles known as WIMPs — weakly interacting massive particles — left over from the Big Bang. Such particles could drift through the Earth like wind through a screen door. Impervious to almost everything except gravity, they would form a shadow universe clumping together into invisible clouds that then attract ordinary matter. Discovering one of them could give a lift to new theories of physics, like supersymmetry, which predicts of a whole new spectrum of so-called superpartners to the particles we already know about, not to mention explicating the nature of more than a quarter of creation.

But until now, the dark matter particles have mostly eluded direct detection in the laboratory or creation at the Large Hadron Collider.

The sky could be a different story. Such WIMPs floating in the halos around galaxies would occasionally collide and annihilate one another in tiny fireballs of radiation and lighter particles, the theorists Michael Turner of the University of Chicago and Frank Wilczek of M.I.T. suggested in 1990.

The Alpha Magnetic Spectrometer is one of the most expensive, complicated and controversial experiments ever mounted in space. Built by an army of 600 scientists from 16 countries, including Italy, Germany, Russia, China and Taiwan, it took 16 years from its approval in 1995 by NASA’s administrator at the time, Dan Goldin, to get to space, on the next-to-last shuttle flight, in May 2011.

By then there were already intimations of dark matter in the heavens. Pamela, a satellite built by Italian, German, Russian and Swedish scientists, registered an excess of anti-electrons, or positrons, in space — perhaps, they said, from collisions of dark matter particles. Using data from the Fermi satellite, researchers at Stanford were also able to detect a similar excess.