IN THE DARK? Studies of spiral galaxies such as Andromeda, pictured here in infrared wavelengths, have provided clues to dark matter's gravitational effects. But more immediate evidence for dark matter's existence, and clues to its true nature, has remained elusive. Image: NASA/JPL-Caltech/UCLA
Most of the matter in the universe remains missing in action—at least, that's long been the standard cosmological paradigm.
Now, however, a small but vocal group of cosmologists is challenging the dark matter tenets of the widely accepted cosmological model, which holds that the universe is composed of roughly 70 percent dark energy, 25 percent dark matter, and only 5 percent normal (or baryonic) matter. Dark matter, whatever it is, exerts a gravitational pull but only interacts with ordinary matter very weakly, if at all, beyond that. Light seems to have no effect on dark matter—hence its name.
Evidence of dark matter's influence on the cosmos stretches back to the 1930s and has only gotten stronger in recent years. NASA's groundbreaking cosmology satellite, the Wilkinson Microwave Anisotropy Probe, has in the decade since its launch delivered a robust indirect detection of dark matter's footprint on the ancient echo of light known as the cosmic microwave background. And dark matter's effects are also inferred in gravitational interactions around clusters of galaxies as well as around individual galaxies themselves.
But the dark stuff itself has yet to be detected, either directly, in particle physics laboratories as a new subatomic particle, via neutrino telescopes also operating in the subatomic realm, or with concrete evidence of such hidden matter using telescopes operating in the electromagnetic spectrum. Some astrophysicists are hopeful that the Fermi Gamma-Ray Space Telescope will deliver corroborating, if still somewhat indirect, evidence for the mutual annihilation of dark matter particles in the galaxy.
"Dark matter comes about because people unquestionably find mass discrepancies in galaxies and clusters of galaxies," says Mordehai Milgrom, an astrophysicist at the Weizmann Institute of Science in Rehovot, Israel.
Stars at the very edges of spiral galaxies, for instance, rotate much faster than can be explained by Newtonian gravity alone; the picture makes sense only if astrophysicists either modify gravity itself or invoke additional gravitational acceleration due to an unknown source of mass such as dark matter.
"The mass of visible matter falls very short of what is needed to account for the gravity shown by these systems," Milgrom says. "The mainstream assumes it is due to the presence of dark matter, while others, like me, think that the theory of gravity has to be modified."
Milgrom's doubts about dark matter have long kept him on the fringe of professional astronomical circles. But as Rutgers University astronomer Jerry Sellwood notes, "people are beginning to think that we should have found some independent evidence for dark matter, and that hasn't happened."
That is arguably largely a result of the fact that dark matter is theorized to interact minimally with normal matter. But some observational campaigns have not seen the effects of dark matter where it is expected to exist. Theory predicts that spiral galaxies, including our own Milky Way, are enveloped by massive dark matter halos that provide the galaxy's missing mass. But the Milky Way's own dark matter halo has also yet to be detected, even indirectly. Its putative existence is primarily inferred from the anomalous rotations of satellite galaxies such as the Magellanic Clouds, which orbit the Milky Way too quickly to be explained by ordinary gravity alone.
More recently, there have also been predictions of a disk of dark matter that would reside in the galactic plane, co-rotating with the Milky Way itself. But in an analysis of the movements of some 300 stars located at least 6,000 light-years beyond the galactic plane, Christian Moni Bidin, an astronomer at the University of Concepción in Chile, and his colleagues conclude that there is "no compelling evidence" for such a dark disk. Given uncertainties in their own analysis, however, they acknowledge that such a disk's existence cannot be completely ruled out.