Here's an excerpt from the BBC's coverage of the Science breakthroughs story --
The "concordance" model of the Universe proposes that over 70% is made up of dark energy, with around 25% composed of dark matter and only 5% of normal matter. In this model, dark matter is constantly being stretched by dark energy.
In February, the Wilkinson Microwave Anisotropy Probe (Wmap) satellite took the most detailed picture yet of the cosmic microwave background - an image of the infant cosmos when it was less than 400,000 years old.
In July, astronomers from the Sloan Digital Sky Survey (SDSS), which aims to map out a million galaxies, published a research paper in which they superimposed their own galaxy-clustering data on Wmap's microwave data.
They claim the results prove that dark energy must exist.
But on Friday (12 December), an international group of astronomers claimed analysis of data returned from the European Space Agency's (Esa) XMM-Newton satellite observatory casts doubt on the existence of dark energy.
The astronomers measured the quantity and energy of X-rays emitted by eight distant galaxy clusters. They say their results may imply that the density of matter in the Universe is very high, contradicting the popular concordance model.
"To account for these results you have to have a lot of matter in the Universe and that leaves little room for dark energy," said Alain Blanchard of the Observatoire Midi-Pyrenees in France.