The US:A ghostly halo detected around a distant cluster of galaxies is the strongest evidence yet for dark matter, the cosmic scaffold around which the planets and stars form, astronomers said yesterday.
The discovery is a milestone in a 70-year search for a substance that has never been seen, yet accounts for nearly all of the mass in the universe.
Because it does not reflect or emit radiation, dark matter has proved impossible to observe directly, even with the most advanced telescopes.
The discovery was announced last night at a Nasa press conference in Washington.
Scientists know there is more to the universe than they can see because the small percentage of the visible universe - stars, planets and clouds of gas and dust - moves as if acted upon by gravitational forces seeming to come from nowhere.
Using the advanced camera for surveys aboard the Hubble space telescope, scientists created a map of dark matter by watching how light from remote stars was bent by gravity as it passed a cluster of galaxies some five billion light years from Earth, in the constellation of Pisces.
The map revealed a ring of dark matter 2.6m light years across, surrounding the galaxies.
The evidence is compelling because it captures the aftermath of cataclysmic collision between two clusters of hundreds of galaxies. The collision knocked the dark matter away from its usual position over the top of the galaxies, allowing scientists to observe its effects on starlight in isolation from other objects that exert a gravitational pull.
"This is the first time we have detected dark matter as having a unique structure that is different from the gas and galaxies in the cluster," said Myungkook James Jee, the team's lead astronomer at Johns Hopkins University, in Baltimore.
"By seeing a dark matter structure that is not traced by galaxies and hot gas, we can study how it behaves differently from normal matter."
The mass of the dark matter was estimated to be equivalent to a staggering 10 million billion suns, each weighing more than 300,000 times that of Earth.
No one knows what dark matter is made of, but astronomers believe it accounts for 80 per cent of the mass of the universe stretching out to form a celestial skeleton around which galaxies form.
The image was captured by training Hubble's camera on a cluster of 300 galaxies for more than 14 hours, using six different filters to observe the glow at various wavelengths. The images have been under analysis since November 2004.
The researchers first suspected the galaxy halo was caused by a flaw in their data. "It took more than a year to convince myself that the ring was real. I've looked at a number of clusters and I haven't seen anything like this," said Dr Jee, whose study is due to be published in the June issue of the Astrophysical Journal.
The team simulated a collision between two clusters of galaxies, and found that while planets within them typically sped past one another, and vast gas clouds of interstellar gas compressed and heated, the dark matter surged into the heart of the collision then rippled back out, slowing under gravity to form a halo.