A 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 today.
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 today 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 5bn 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 a 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 star light 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% 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 1 issue of the Astrophysical Journal.
Using computer models, 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 and then rippled back out, eventually slowing under gravity to form a halo.
Scientists have pondered the existence of dark matter since 1933, when the Swiss astronomer Fritz Zwicky claimed that a distant cluster of galaxies would fall apart were it not for the extra gravitational pull of a mysterious invisible mass, which was later dubbed dark matter.
Astronomers verified his findings by showing that stars swirling around distant galaxies zip around too fast.
Richard White, a member of the team at the Space Telescope Science Institute, in Baltimore, said the findings could spell the end for alternative theories on dark matter, which seek to explain the missing mass of the universe by tweaking the strength of gravity. "The fact that we see the ring by itself makes this very strong evidence for dark matter. This is a really key element in our understanding of what this dark matter might be," said Dr White.
The elusive nature of dark matter has not held scientists back from speculation. Some believe it consists of black holes and dead stars, collectively named massive astrophysical compact halo objects, or Machos. Others say dark matter is made of exotic particles called weakly interacting massive particles, or Wimps.
In the 1970s Frank Wilczek, a Nobel prize-winning physicist at Massachusetts Institute of Technology, named such particles Axion, after a brand of washing powder since they cleaned up a long-standing problem in theoretical physics.
Other scientists favour the neutralino particle as the main constituent of dark matter. Although each particle is likely to have a mass far greater than an electron, clouds of dark matter do not notice other material in space and probably waft through the Earth and other planets unnoticed, Nasa scientists believe.
Understanding the nature of dark matter has become one of the most pressing goals for 21st century physics. "There's no reason why dark matter has to be made up of one single type of particle, but we tend to think of it that way because we'd prefer to have one mysterious thing rather than many mysterious things," said Dr White.
In January, a team of scientists lead by the British astronomer Richard Massey, at the California Institute of Technology, published the first 3D map of dark matter, which revealed how it clung around galaxies and held clusters of them together.
"This is a really exciting result and everyone wants this so much, but the burden of proof has to be really heavy," said Dr Massey. "We know that as the Hubble space telescope goes in and out of the shadow of the Earth on every orbit, it heats up and cools down and expands and contracts as if it's breathing. That is enough to change the optics slightly and produce spurious effects. I really want to see some more observations and see this effect reproduced."
Tragically, the Hubble camera which took the latest dark matter images malfunctioned at the end of January, leaving scientists no way of obtaining further images. Nasa plans to fix the ageing telescope. Dr Massey said: "It's heartbreaking the camera failed just as we're figuring out how to do this."