About 90 per cent of the universe's mass cannot be accounted for by telescopic obser-vations, which is why it is called "dark matter", and the search has begun to find out what it is.
One theory is that it is composed of large, Jupiter-like planets which do not emit radiation and so cannot be seen. Another more likely theory is that it is composed of invisible sub-atomic particles that are not easily detected. Scientists have set up instruments in Europe's deepest mine-shaft at Boulby, Cleveland, to detect them as they pass through the earth.
Neil Spooner, a physicist at the University of Sheffield, said he believes the dark matter is made up of weakly interacting massive particles.
These are so numerous that billions of them pass through a person each second, he said.The detectors show "bumps" that do not have a conventional explanation, such as background "noise". He called these "unexpected events".