Over the last decade or so, astronomers have come to an unsettling realization: they don’t know what at least 90 percent of the universe is made of. From observing clusters of galaxies, astronomers know that the gravitational pull of visible matter can’t account for all the motions they see within the clusters. Thus some type of matter they can’t see--called dark matter--must be out there. Tony Tyson, an astronomer at Bell Labs in Murray Hill, New Jersey, recently made the first detailed map of dark matter’s distribution within a cluster of galaxies. His map may not reveal what dark matter is, but at least it may show what it’s not.
To make his map, Tyson used a sort of cosmic telescope with a lens more than a million light-years across. We used the Hubble Space Telescope as the eyepiece of a very large telescope, says Tyson, in which the objective ...
