Five years ago, the Nobel Prize in Physics was awarded to three astronomers for their discovery that the universe is expanding at an accelerating pace. This led to the widespread acceptance of the idea that the universe is dominated by a mysterious substance named 'dark energy' that drives this accelerating expansion. Now, a team of scientists has cast doubt on this standard cosmological concept. The evidence for acceleration may be flimsier than previously thought, they say, with the data being consistent with a constant rate of expansion.