INVISIBLE OR NONEXISTENT? A composite image of the galaxy cluster Abell 520 shows the inferred presence of matter, primarily dark matter, in blue.
X-ray: NASA/CXC/UVic./A.Mahdavi et al.; Optical/Lensing: CFHT/UVic./A.Mahdavi et al.

Theorists and observational astronomers are hot on the trail of dark matter, the invisible material thought to account for puzzling mass disparities in large-scale astronomical structures. For instance, galaxies and galactic clusters behave as if they were far more massive than would be expected if they comprised only atoms and molecules, spinning faster than their observable mass would explain. What is more, the very presence of assemblages such as our Milky Way Galaxy speaks to the influence of more mass than we can see. If the mass of the universe were confined to atoms, the clumping of matter that allowed galaxies to take shape would never have transpired.

Dark matter was theorized into existence to account for the missing mass. The prevailing view holds that dark matter contributes five times as much to the mass of the universe as ordinary matter does.

But some researchers have taken to approaching the problem from the other direction: What if the discrepancy arises from a flaw in our theory of gravity rather than from some provider of mass that we cannot see? In the 1980s physicist Mordehai Milgrom of the Weizmann Institute of Science in Rehovot, Israel, proposed a modification to Newtonian dynamics that would explain many of the observational discrepancies without requiring significant mass to be hidden away in dark matter. But it fell short of describing all celestial objects, and to incorporate the full span of gravitational interactions, a modification to Albert Einstein's theory of general relativity is needed.

A review article in the November 6 Science checks in on the status of these modified-gravity theories, including a proposal put forth by physicist Jacob Bekenstein of The Hebrew University of Jerusalem in 2004. Pedro Ferreira, a University of Oxford cosmologist and one of the review paper's co-authors, says that there is good news and bad news for proponents of such models.

The bad news is that in order for modified versions of general relativity to work, some sort of unseen—or "dark"—presence must be in play, which in some cases can look a lot like dark matter. "If you try and build a consistent, relativistic theory that gives you modified Newtonian dynamics, you have no choice but to introduce extra stuff," Ferreira says. "I don't think it will be described by particles, in the way that dark matter is described—it may be described in a more wavelike form or a more fieldlike form."

In other words, a theory of gravity can do away with dark matter but cannot describe the universe simply as the product of a tweaked Einsteinian gravity acting on the mass we can see. "The old paradigm where all you were doing was modifying gravity simply doesn't hold," Ferreira says. "You modify gravity, but through the backdoor you introduce extra fields, which means that the distinction between dark matter and modified gravity isn't as clear as people thought before."

The good news? According to Ferreira, "all is not lost." Observational campaigns now in the works, such as the Joint Dark Energy Mission planned by NASA and the U.S. Department of Energy as well as an international radio telescope project known as Square Kilometer Array, should allow astronomers and cosmologists to test competing worldviews in the next decade or so.

By cross-correlating large-scale surveys of galaxies and observations of how galaxies distort background light in a relativistic process known as weak lensing, Ferreira says, the true nature of mass and the forces acting on it can be tested. "Whether gravity is modified or not will greatly affect the result," he predicts.

Although Ferreira works on theories of modified gravity, he is careful to note that the new paper does not advocate for those theories' correctness over the prevailing model. In his personal view, Ferreira says, "by far the simplest proposal is normal gravity plus dark matter."