Dark Matter in Galaxy Clusters
Some of the most dramatic evidence of dark matter shows up in images of large clusters of galaxies. The gravitational pull of matter in the cluster bends and twists the light from more distant galaxies, producing a plethora of strange optical effects ranging from distorted arcs to multiple images of the same background object. This process is called gravitational lensing. The intensity of the distortion indicates the strength of the overall gravitational field, and hence the total mass, of the lensing cluster.
The mass of galaxy clusters inferred from lensing studies is about 10 times as great as that of all their stars combined with the hot X-ray-emitting gas that fills the space between the galaxies. The excess mass must be dark matter. (That conclusion assumes gravity behaves the same on cosmic scales as it does on Earth—a logical but untested article of faith. See “Nailing Down Gravity,” Discover, October 2003.) In clusters, too, dark matter seems to form a massive halo around the bright parts. Earlier this year, Jean-Paul Kneib of Caltech and his colleagues analyzed gravitational distortions produced by a galaxy grouping named CL0024+1654. The resulting map, pictured below, exposes the location of dark matter in and around that cluster.
So what makes up all this dark material holding together clusters of galaxies? Burned-out or failed stars cannot account for nearly enough mass. In fact, there are strong reasons to think that much of the dark stuff is not made of atoms at all. The Big Bang theory makes detailed predictions about the total number of ordinary atoms and about the relative abundance of deuterium (heavy hydrogen) and helium in the universe. These predictions match the observed cosmic composition if the modern universe has an average density of 0.2 hydrogen atoms per cubic meter—far less than the amount of matter seen around galaxies and galaxy clusters.
As a result, astronomers are convinced that most dark matter must consist of particles that do not influence nuclear reactions. That rules out all atoms but allows many other types of elementary particles. The most plausible of these dark matter candidates are neutrinos. These ghostly particles are so unreactive that trillions of them pass through you each second without disturbing any of the atoms in your body. They then continue right on through Earth, with absolutely no effect.
Theoretical calculations indicate that there should be as many as 100 million neutrinos for every atom in the universe. Because neutrinos are so abundant, they could be the dominant dark matter even if each weighed only a tiny fraction as much as an atom. Until recently, physicists thought neutrinos carried no mass at all, but studies completed in 1998 at the Super-Kamiokande detector in Japan indicate otherwise. The inferred mass is so slight, however, that neutrinos cannot account for all dark matter. At most, they could match the mass of the stars. Adding up all the dark forms of ordinary matter (gas clouds, brown dwarfs, black holes, and so on) still leaves 95 percent of the mass in the universe unaccounted for.