Somewhere out in deep space there may exist a galaxy that looks identical to ours. It would have the same spiral arms, stars and gas clouds. The only difference would be that it was made of anti-matter, not normal matter.

Now, in an attempt to capture anti-matter from the stars, Nasa has launched a 60-storey-high balloon to the upper fringes of Earth's atmosphere.

Matter and anti-matter are opposites, bring them together and they annihilate each other in a burst of energy. Leave it on its own and it behaves just the same as our kind of matter, except that the electrical charge of sub-atomic particles such as the electron are reversed.

The equipment swings out as the balloon launches

But that does not really matter, the anti-matter universe would be fundamentally the same as this one.

But for some reason our Universe consists completely, or almost completely, of ordinary matter and not a combination of matter and anti-matter. Why? And could any of the distant galaxies we see in space be made of anti-matter.

Sky high

The balloon is Nasa's largest ever, over a million cubic metres in volume and lifted off from Lynn Lake, Manitoba, Canada, last week. Its 38-hour flight took place more than 32 kilometres (20 miles) above Earth. Its mission - to use a Japanese-built instrument to look for evidence of anti-matter galaxies.

It is tracking cosmic rays which are actually high-energy particles that traverse the universe.

The Bess project (Balloon-borne Experiment with a Superconducting Solenoidal magnet) is led by Professor Shuji Orito of the University of Tokyo, and is sponsored in the US by Nasa and by Monbusho in Japan.

The detector is extremely sensitive

"We have collected excellent data, which should contain several hundred antiprotons among a hundred million cosmic-ray particles that passed through our detector," said Professor Orito.

Although most scientists believe that the Universe is made of "ordinary" matter, some speculate that antimatter galaxies may exist. However, no evidence of these galaxies has been found.

Anti-molecules

Previous balloon flights have detected anti-protons but if Bess were to find a more sophisticated form of antimatter, such as molecules of anti-helium, it would provide evidence that antimatter galaxies exist.

"The discovery of anti-helium would be stunning," said Professor Orito.

"We have actually found no anti-helium in data taken during five flights from 1993 to 1998, while we have detected three million helium nuclei. This fact provides the most direct evidence that the Galaxy and the nearby part of the Universe are made solely of matter, not antimatter."

Rewrite the textbooks

The detection of anti-helium would rewrite the books on cosmology, according to Dr Jonathan Ormes, head of the Laboratory of High Energy Astrophysics at Nasa's Goddard Space Flight Center.

"The idea that large regions or domains of the Universe might be built of antimatter has been discussed for many years," said Dr. Ormes.

The instrument's sensitivity has improved dramatically since it was first flown in 1993, when it made the first unambiguous detection of cosmic antiprotons. After each flight the Bess team improves the instrument for the next flight, resulting in a steadily increasing number of detections.