By Jonathan Amos
BBC News science reporter
Last Updated: Sunday, 5 February 2006, 23:25 GMT
Astronomers have for the first time put some real numbers on the physical characteristics of dark matter.
This strange material that dominates the Universe but which is invisible to current telescope technology is one of the great enigmas of modern science.
That it exists is one of the few things on which researchers have been certain.
But now an Institute of Astronomy, Cambridge, team has at last been able to place limits on how it is packed in space and measure its "temperature".
It's the first clue of what this stuff might be," said Professor Gerry Gilmore. "For the first time ever, we're actually dealing with its physics," he told the BBC News website.