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AS IT is, dark matter is stuff we cannot see. So astronomers were stumped when some of it went missing two years ago. Now it has turned up right where it鈥檚 supposed to be, much to the relief of those who thought they might have some explaining to do.

The existence of dark matter was suggested in the 1970s when astronomers found that the gravity of visible matter alone was not enough to keep the fast-moving stars and gas in spiral galaxies from flying out into space. They attributed the extra pull to dark matter, which is now thought to outweigh normal matter by 6 to 1 and probably seeded the first galaxies.

So it came as something of a shock in 2003 when Aaron Romanowsky, then at the University of Nottingham, UK, discovered three 鈥渘aked鈥� elliptical galaxies devoid of dark matter. Planetary nebulae 鈥� shells of gas formed by dying stars 鈥� in the outer reaches of these galaxies were moving too slowly for the galaxies to contain much dark matter. Now, Avishai Dekel of The Hebrew University of Jerusalem, Israel, and his team have shown how the gas could be moving slowly even with normal amounts of dark matter in the galaxy.

鈥淣ebulae in the outer reaches of some galaxies were moving too slowly for the galaxies to contain much dark matter鈥�

Astronomers believe elliptical galaxies form when spiral galaxies merge. When Dekel and his team modelled 10 such mergers, they found that some stars were sent on paths that flung them through the centre of the galaxies and far out again in thin, elongated orbits. So such stars would spend the most time near the remote ends of their orbits, where they are moving slowest. Over billions of years such stars might evolve into the kind of slow-moving nebulae Romanowsky found.

The work is 鈥渧ery elegant鈥�, says Vera Rubin, of the Carnegie Institution in Washington DC, who pioneered the study of dark matter in spiral galaxies. But Romanowsky is not yet convinced. He says Dekel鈥檚 simulations may represent a special class of merger that just happens to produce elliptical galaxies that mimic others with little or no dark matter. 鈥淭he results still suggest to us that there is an issue with the distribution of dark matter in elliptical galaxies,鈥� he says.

Michael Merrifield, a member of Romanowsky鈥檚 team at the University of Nottingham, says that if it is true that some elliptical galaxies don鈥檛 have much dark matter, 鈥渋t is a very strange result鈥�. But so far none of the existing theories can explain how some elliptical galaxies have managed to shed their share of dark matter, leaving just the stars, he says. 鈥淪o it may require some radical rethinking of how galaxies form.鈥�

Dekel鈥檚 paper appears at . His team declined to speak to 快猫短视频.