As understanding ofthe 100,000 million galaxies spreading out from each other to fill acontinually expanding space increases, cosmologists are still searching for the mysterious dark matter which is thought to give space more than 90 per cent of itsmass and account for much of the universe's observed behaviour.

Much is speculative. ButSir Martin Rees, professorof astronomy at Cambridge University, says there is good quantitative evidence totrace the observable universeback to its earliest stages.

He adds that astronomers can actually see the remote past and as such havean advantage over palaeontologists, whose inferences depend on fossil traces.

For Chris Done, x-ray astronomer at Durham University, what is challenging is the learning of fundamental new physics.

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Other planetary systems,similar to our own, have now been discovered. "(Thesediscoveries) suggest that forming planets around astar cannot be that difficult to do," says Done. "Stars are formed from clouds of gas. Gravity pulls in material which gets hotter and hotter until you get a nuclearreaction and the star forms.

"The planets we have found around the stars are more or less in the same plane. Itprovides us with evidence which might suggest that planets form from a disk of material around a star."

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According to Durhamcosmologist Richard Bower,it would be the discovery of dark matter - heavy particles which are not electrically charged and cannot be seen, but exert mass and gravitational pull - which would be the biggest breakthrough.

"More than 90 per cent of the universe we don't really know about. We think it's there, but we don't see it," he said. "How much dark matter is there in this room? - we just don't know."