an “Einstein ring.” The radius of the ring depends on the gravitational distortion of the matter bending the light. Therefore, it provides a means of gauging how much mass lies in the light rays’ path.

Pacyznski adapted this method to account for minute concentrations of mass, showing that a customized form of gravitational lensing would be a suitable way of discerning small, otherwise invisible, bodies in galactic halos or elsewhere. The microlensing effect would be much more subtle than large-scale lensing—involving a slight brightening and dimming as the MACHO passed by—but still potentially discernible by computer-aided instruments.

Using gravitational microlensing to identify intervening MACHOs is akin to employing an eye chart to find the best-fitting glasses. In optometry, if an image looks distorted, the cause of such blurriness can be inferred and then corrected. Similarly, in astronomy any change in the appearance of a star could indicate the fleeting distortion caused by an intervening object. Working backward from the image, we can deduce the properties of the unseen agent, particularly its mass and size.

Armed with this powerful technique, several teams of astronomical detectives set out to sleuth for MACHOs in the early 1990s. One collaboration, headed by Charles Alcock and simply called the “MACHO group,” conducted an extensive scan of millions of stars in the Large Magellanic Cloud (LMC)—a small satellite galaxy of the Milky Way. Another team, called EROS (Expérience de Recherche d’Objets Sombres—French for “The Experimental Search for Dark Objects”) focused on both the LMC and the Small Magellanic Cloud—scanning a similarly vast array of stars. Each group looked for evidence of brightness variations, caused by invisible objects passing between the stars and Earth.

The immense numbers of stars needed for these surveys derived from the tremendously low odds that any one of them would be lensed by a MACHO. Both the star and the MACHO would need to line up almost perfectly (within one milliarcsecond of angle, to be