WASHINGTON: Astronomers say they have recorded the first direct observation of the mysterious substance known as dark matter.
"This is really exciting, in my opinion," said Sean Carroll, a cosmologist at the University of Chicago. "Once and for all we can say dark matter does exist."
Dark matter does not consist of electrons, protons and the other building blocks of ordinary matter. If a "cloud" of dark matter swept through the solar system, for example, it would pass through the Earth, the sun and every other concentration of ordinary matter without "touching" them, theorists said.
But because dark matter does not give off light or absorb light or interact with ordinary matter it cannot be directly observed.
Astronomers had believed in its existence because ordinary matter does not provide enough gravity to hold galaxies together.
To prove the existence of dark matter, scientists from Harvard University and the University of Arizona used images from NASA's Chandra X-ray Observatory to photograph two galaxy clusters that appear to have collided at high speed about 100 million years ago.
Enormous clouds of intergalactic gas were heated to nearly unimaginable temperatures by the collision, causing them to give off the X-rays, the astronomers said.
Theories suggest that such a collision would act like a filter. The ordinary matter in the galaxy clusters would be slowed down, but the non-interactive dark matter would pass right through, unimpeded.
Sure enough, by analysing how light from even more distant galaxies was distorted by the collision, the astronomers mapped huge clouds of something that extended past the two clusters on either side.
"Dark matter is the only explanation for the presence of this matter," said a Harvard astrophysicist, Dr Maxim Markevitch. "This is the first time we've actually seen a separation in space of dark matter and ordinary matter."
Dr Carroll said: "Now we must go out and find out what dark matter is and where it came from and why there's so much of it."