Moni Bidin, the lead author on a paper detailing the finding in the November 20 issue of The Astrophysical Journal Letters, says that one can always conclude that dark matter escapes detection because it has an exotic nature or unexpected properties. "But failing to detect it in indirect kinematical measurements such as ours," he says, "means finding a way out is harder."
Another dynamical complication comes from the so-called Tully-Fisher relationship, which describes the relation between a galaxy's luminosity and its rotation velocity: the higher the luminosity, the faster a galaxy rotates.
The measured rotation speeds on the outskirts of a spiral galaxy, Milgrom says, depend in "a very strict manner only on the total visible mass of the galaxy." But if the theory of dark matter is correct, then the speed of stars rotating on the galaxy's outskirts should also depend on the shape of the galaxy's dark matter halo.
"Dark matter halos should be lumpy, underinflated football shapes; not spherical," says Stacy McGaugh, an astronomer at the University of Maryland, College Park. "Statistically, that means we should see many [different galactic rotation] velocities for the same luminosity. We don't."
Instead, McGaugh says, the "baryonic tail wags the dark matter dog." In other words, astronomers can predict just what the galactic rotation curves will be from a given galaxy's stellar distribution. McGaugh makes the claim that if dark matter is dominant, observers shouldn't be able to predict the galactic rotation curves by what they see in normal luminous matter.
"Because each dark matter halo should be unique, you should see lots of variation in rotation curves for the same galaxy," he says. "You don't expect the kind of uniformity that we observe in hundreds of galactic rotation curves."
Even if dark matter raises questions on such large galactic scales, particle physicists are hopeful that it will be detected in the lab. If dark matter particles in the sun, for instance, undergo self-annihilation, then such annihilation events could create high-energy neutrinos that would potentially be detectable with ground-based neutrino telescopes.
Then there are detectors, such as the Xenon100 experiment at Italy's National Laboratory in Gran Sasso, built to register direct hits from particulate dark matter. Xenon100 is designed to search for the most favored dark matter particle candidate—the weakly interacting massive particle (WIMP)—by watching for signs that a WIMP has recoiled off an atom in a tank of liquid xenon. A recent analysis of an 11-day observing run in 2009, however, failed to identify any such dark particles, casting doubt on two competing groups' prior claims of possible dark matter signals.
One problem in making such detections is the uncertainty over dark matter's density in the local universe, says Chris Mihos, an astrophysicist at Case Western Reserve University. "Does the dark matter particle not exist," he wonders, "or are we just unlucky in terms of the local dark matter density?"
Current direct detection scenarios include potential dark matter particles with masses between one and 1,000 times the mass of a proton and with interaction "cross-sections" roughly one trillionth the size of a neutron.
After each non-detection, McGaugh says, theorists continually redefine the interaction cross-section of WIMPs to safely undetectable levels. This kind of behavior, he adds, can spark a never-ending game of leapfrog between experimental physicists and theoreticians, allowing them to continue business as usual without ever revising their cosmology.
"There is a lot of misplaced certainty in the dark matter model—a feeling that it's not 'if' we directly detect dark matter, but 'when,'" Mihos says.
Or, as McGaugh puts it, "Once you convince yourself that the universe is full of an invisible substance that only interacts with ordinary matter through gravity, then it is virtually impossible to disabuse yourself of that notion. There is always a way to wiggle out of any observation."