Crisis averted: Dark matter was there all along

Fans of dark matter can rest easy. A study published last month raised eyebrows by suggesting that our cosmic neighbourhood is empty of the extra mass needed to hold the galaxy together. But a re-analysis shows that the dark matter was there all along.

Dark matter is the mysterious, invisible stuff that makes up 83 per cent of the matter in the universe. It is responsible for keeping galaxies from flying apart despite their high spinning speeds, and has aided our understanding of how structures in the universe formed.

The most popular theories say that dark matter is a hitherto undetected particle called a WIMP (weakly interacting massive particle) that is shy of interacting with ordinary matter through any force except gravity.

But several underground detectors waiting for WIMPs have come up empty, or with conflicting results. If the galaxy is so full of dark matter, why hasn't it shown up yet?

In April, a team led by Christian Moni-Bidin of the University of Concepcion in Chile thought they had a solution: the WIMPs aren't actually there.

The team tracked the motions of more than 400 stars within 13,000 light years of Earth to estimate the mass of matter ? visible and dark ? in the sun's local neighbourhood. They concluded that the mass they found could be explained by the visible matter alone, with no need for dark matter.

But the team made a subtle error, say Jo Bovy and Scott Tremaine of the Institute for Advanced Study in Princeton, New Jersey.

Moni-Bidin and colleagues considered stars whose orbits take them far above or below the Milky Way's main bright disc, and used the speed at which they orbit the centre of the galaxy to figure out how much of a pull they feel from the nearby mass of stars and dark matter. They assumed that the stars' speeds would be the same no matter how far they were from the galactic centre. Observations of dust clumps have shown that this assumption is true for young stars orbiting in the galactic disc, which mostly move in a near-perfect circle.

But the stars that orbit high above or far below the disc can't have circular orbits, Bovy says. The only stars that reach such great heights have been kicked away from the disc by matter in the galaxy's spiral arms, which sent them on highly elliptical orbits.

This means that their speeds are not the same at all distances from the galactic centre. On average, Bovy and Tremaine found, their rotational speeds should be slower than assumed by Moni-Bidin and his colleagues.

"By assuming that stars rotate at the same velocity no matter at what distance they are from the centre of the Galaxy, they underestimated the total amount of matter in the solar neighbourhood and they concluded that there was no room for dark matter," Bovy says.

Bovy and Tremaine re-analysed the data and found that the amount of dark matter in the sun's neighbourhood agrees with previous predictions ? if anything, there might be a little more nearby dark matter than we thought before.

That means that "the prospects of detecting dark matter on Earth are good", Bovy says. "We are now sure that the local dark matter density is what experimental particle physicists have long assumed when running their experiments."

Moni-Bidin says he is aware of the paper, but declined to comment until it has gone through peer review. The paper has been submitted to the Astrophysical Journal.

But dark matter's true believers were never really worried. Dan Hooper of Fermilab in Batavia, Illinois, was sceptical of the original study from the beginning.

"The evidence is going to have to be very strong before you dislodge the long-held opinion that there's a lot of dark matter nearby," he says.

Reference: A copy of Bovy and Tremaine's paper is available on the arxiv preprint server

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