Mundane dark matter may lurk in starry clusters

DARK matter - the mysterious substance thought to make up about 80 per cent of the universe's matter - could be more mundane than thought. Inside balls of stars known as globular clusters, at least.

Unless we have misunderstood gravity, dark matter must be there - holding rotating galaxies together. But we don't know what it is. Initially, it was thought to be planets and stars too dim to be seen directly. Such objects would reveal themselves when they pass in front of bright stars, distorting the image with their gravity, but the objects turned up by such "microlensing" searches in our galaxy have not revealed nearly enough matter. So it is assumed that dark matter is something more exotic, such as novel theoretical particles.

Now, Pawel Pietrukowicz of Warsaw University in Poland and colleagues have spotted a tiny star in the globular cluster M22 acting as a lens for a background star. At just 0.18 times the sun's mass, it is the smallest star ever seen in a globular cluster. Because its effects on the larger star were seen after just 10 weeks of observations, the team says there are probably many more like it in the cluster, perhaps even enough to account for all the dark matter needed to hold the cluster together. The work will appear in The Astrophysical Journal Letters.

While exotic dark matter is still needed outside of globular clusters, knowing that it might not be needed in this one, and perhaps others like it, could give clues to the stuff's properties. Turn to page 30 for more on dark matter

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