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Dark matter hides light from lonely stars

Light riddle Halos of mysterious dark matter could be hiding large numbers of unseen stars on the fringes of galaxies, according to a new study.

The discovery, reported in the journal Nature, could explain why there's more light in the universe than there are stars to produce it.

"Existing explanations such as very distant far away galaxies and nearer very faint galaxies can't explain this discrepancy," says lead reseracher Professor Ned Wright of the University of California.

"The idea of not-so-far-away faint galaxies (producing this light) is off by a factor of about ten, while the 'distant galaxies' hypothesis is off by a factor of about a thousand."

In large clusters of galaxies, astronomers have found much higher percentages of this light discrepancy, as high as 20 per cent.

To solve the riddle, Wright and colleagues used NASA's Spitzer infrared space telescope to map a region of the sky ten billion light years away in the constellation Boötes.

Blocking out the light

"There were a large number of galaxies in that field, so we cut out the light from those galaxies by masking them," says Wright.

"We ended up with a picture of the sky, with all the galaxies cut out which you expect to be perfectly uniform and dark, but we still see small amounts of light coming from somewhere."

Wright and colleagues measured this unexplained light, which they call fluctuations, and found it appeared to be coming from stars hidden in the halos of dark matter surrounding galaxies.

"Galaxies are collections of stars that sit inside a more extended halo of dark matter," says Wright.

"We can't see the dark matter but we know it's there because of its gravitational impact on the galaxy."

Flung out by gravity

Wright and colleagues, contend that about one star in every thousand are flung out of their galaxies by the gravitational perturbations produced during galactic mergers.

"When these galaxies merge not all of the stars end up in the merged galaxy," says Wright. "It's these stars, which end up hidden in the dark matter, that are providing the additional light we're seeing."

He says he always felt galaxies were a lot fuzzier than expected.

"This fuzziness means there no sharp edge to the galaxy boundary."