An artist’s impression of the predicted dark matter distribution (shown in blue) around the Milky Way. See more here. (Image: L. Calçada/ESO)





Video: Blue halo shows dark matter around Milky Way

Invisible dark matter is supposed to make up over 80 per cent of the universe’s mass but a new survey of nearby stars and galaxies suggests our patch of the cosmos could be totally free of the stuff. “The claim is that in this volume, there is no dark matter,” says Christian Moni Bidin at the University of Concepción in Chile, who led the study.

The surprising finding contradicts otherwise successful predictions about the distribution of dark matter in the universe, leaving many puzzled about how else to explain the universe’s history. It also fits with new observations by a fiery minority of physicists who dispute whether the mysterious matter, which has never been observed directly, is even necessary.


Rather than seeing it directly, physicists first deduced the existence of dark matter from the way that our galaxy rotates. If the only matter in the Milky Way is the visible stuff like stars and planets, then stars at the edge are moving too quickly to be held by our galaxy’s gravity.

In order to keep the stars on the fringes from flying away, there must be some additional mass creating the extra gravity needed to hold them. A similar situation has been observed in other galaxies and physicists have used this calculate that this must amount to about 83 per cent of the total mass of the universe.

Ant on vinyl

This has in turn provided predictions for the way that dark matter is distributed in the universe, and fed into our understanding of how the universe formed. For example, supercomputer simulations of the entire history of the universe that include these distributions of dark matter do an excellent job of reproducing the structures we actually see in the universe now.

However, until now most velocity measurements considered only stars zipping around the Milky Way radially, like an ant sitting on a vinyl record. By contrast, Moni Bidin’s team looked outward from the plane of the galaxy, perpendicular to the galactic disc. Using historical survey data and new observations from telescopes at the La Silla Observatory and the Las Campanas Observatory, both in Chile, the researchers mapped the motions of more than 400 stars up to 13,000 light-years from the sun.

They used those measurements to calculate the mass of matter in a volume four times larger than had been considered before at this level of precision. Under the standard dark matter theory, there should be at least as much dark matter as visible matter in this region. “Contrary to our expectations, there is none,” Moni Bidin says. “The result matches the visible mass strongly.” The work will be published in the Astrophysical Journal.

That might explain why experiments on Earth hoping to catch particles of dark matter have turned up confusing results, but it’s too soon to give up on dark matter, says dark matter theorist Dan Hooper at Fermilab in Batavia, Illinois. He says we still need that extra mass to explain why the galaxy holds together – and how all the structures in the universe, from dwarf galaxies to superclusters, formed at all.

Snowballing clusters

“We have many independent lines of reasoning that lead us to the conclusion that we have substantial amounts of dark matter in the local part of our galaxy,” says Hooper. “This is not going to be easily abandoned as an idea. I’m not saying they’re wrong, just that you’re going to have to work really hard to convince me.”

Moni Bidin’s colleague Rory Smith, also at the University of Concepción, agrees that dark matter is still needed. “It explains an enormous number of things really famously, and puts them together into one framework,” he says. “That’s a really powerful theory.”

However, the new survey is not the only example of observations that do not fit with our current picture of dark matter. In a recent study, to be published in Astrophysical Bulletin, Igor Karachentsev calculated that much less dark matter than expected was required to explain the amount of mass in the local universe, a region 163 million light years from the sun in every direction. That takes the headache posed by Moni Bidin’s work and “makes it even worse”, Moni Bidin says.

Meanwhile, Pavel Kroupa at the University of Bonn in Germany has looked at the way dark matter around the Milky Way might be distributed. In the standard view, dark matter drew together under its own gravity to form small clusters shortly after the big bang. Those clusters snowballed in size, and galaxies as we see them today grew up inside massive, near-spherical haloes of dark matter.

Einstein alternative

If that were true, the streams of stars, clusters and small galaxies that orbit the Milky Way should be distributed randomly in a sphere around the main disc. But Kroupa reports in a paper to be published in Publications of the Astronomical Society of Australia that most of them are clustered in an enormous disc that rotates in a plane perpendicular to that of the Milky Way. That disc could be the remnants of another galaxy that collided with the Milky Way some 11 billion years ago, but it could not be the result of dark matter, Kroupa says.

One explanation for all three anomalies is an existing, alternative explanation for the high speeds of stars on the outer edge of the Milky Way, known as Modified Newtonian Dynamics (MOND). Supported by a significant minority of physicists, including Kroupa, MOND assumes that gravity works differently at cosmological scales than it does on the scales we are used to.

Most physicists however think dark matter is still too successful to abandon, and that MOND is too immature to act as a replacement. “MOND has done an incredible job explaining rotation curves really accurately,” Smith says. “But at how well it’s been tested, at the moment it’s not in a place to replace dark matter.”

References: Moni Bidin et al: arxiv.org/abs/1204.3924 (Astrophysical Journal, in press); Karachentsev: arxiv.org/abs/1204.3377 (Astrophysical Bulletin, in press); Kroupa: arxiv.org/abs/1204.2546 (Publications of the Astronomical Society of Australia, in press)

The ongoing WIMP war Even for dark matter’s biggest fans, there’s trouble brewing. Labs across the world are waiting with detectors wide open for hypothetical dark matter particles called WIMPs (weakly interacting massive particles), but what they are seeing is not easy to explain. The “WIMP wars” have raged since 1998, when the DAMA experiment in the Gran Sasso lab in Italy claimed its detector was sparkling with particles that could be WIMPs. The team reported the same in 2008, but no other experiment had seen anything. The plot thickened in 2011, when two other experiments – CRESST II, also in Gran Sasso, and CoGeNT, housed in the Soudan mine in Minnesota – reported flickers of dark matter in their detectors, too. That would have made a pretty strong case, if not for two similar experiments that have seen no dark matter at all. Xenon100, a tub of liquid xenon in Gran Sasso, and CDMS II, next door to CoGeNT, have so far come up empty. CDMS presented its most recent disappointing results in February at the Dark Matter 2012 conference in Los Angeles. Subsequently, Juan Collar, a member of the rival CoGeNT team at the University of Chicago noticed that their results only considered WIMPs above a certain energy. When he re-analysed the CoGeNT data for WIMPs of lower energies too, he found a signal that might be consistent with particles of dark matter (arxiv.org/abs/1204.3559). Sadly, the signal still differs from the detections by the other groups, so Collar is not announcing evidence for dark matter yet. “I’m personally not ready to believe that we’re seeing WIMPs,” he says.