Every so often a scientific discovery comes along that is so profound and significant that it has "Nobel prize" stamped clearly across it. That is surely the case for the discovery made 10 years ago by two teams of scientists that the expansion of the universe is accelerating. The discovery — made by members of the High-Z Supernova Search Team and the Supernova Cosmology Project (SCP) — points to the existence of a weird, gravitationally repulsive "dark energy" that is driving the acceleration and may account for some 75% of the entire mass–energy content of the universe.

The problem is that no-one knows what this dark energy is or whether it is changing with time. The most conventional explanation is that dark energy is some kind of "cosmological constant" that arises from empty space having a non-zero "vacuum energy" due to the spontaneous creation and annihilation of elementary particles. But if that is the case, then there must be something fundamentally askew with our knowledge of particle physics: given the particles we know exist, the vacuum energy density is then 10120 times larger than the value needed to account for the observed cosmic acceleration.

This issue of Physics World marks the 10th anniversary of the discovery of the accelerating universe by examining how our understanding of dark energy may develop over the next decade. Saul Perlmutter, the astrophysicist who headed the SCP team, is upbeat about our prospects of solving the mystery of dark energy, which will be the focus of numerous projects set to come to fruition in the next few years (see "Dark energy: the decade ahead"). Cosmologist Lawrence Krauss, however, is more gloomy: he fears that experiments may never be able to tell us if dark energy is a cosmological constant or something more exotic (see "A dark future for cosmology").

But just as fascinating as the science of the accelerating universe is the story of the discovery itself, which involved two fiercely competitive groups. As Robert P Crease describes (see "Dark energy"), it is not easy to be sure when the discovery was made, to whom it should be credited, and even how it happened. Although the two groups mulled over the first hints of cosmic acceleration in autumn 1997 in private discussions and in early 1998 at public meetings, outsiders will probably be more comfortable in thinking the discovery "occurred" when the refereed papers were published — September 1998 in the case of the High-Z team's paper and June 1999 for the SCP team. But is that fair? Perhaps the discovery occurred when the papers were posted online, which was several months earlier in both cases. Or did it occur when someone first told the media about it or first wrote it down in their notebook? Then there is the issue of which team's analysis was fuller or more complete.

Assigning credit for a scientific discovery is never easy, especially when two rival, interacting teams of scientists are involved. Some members from both teams have been particularly worried about Crease's article, which is one of the first attempts by a historian to examine this discovery in detail and went through over 20 drafts. Crease found it hard enough to present this hornet's nest of a story as fairly as possible within the constraint of a short magazine article. But what his article reveals is how deeply scientific progress is indebted to ambition, desire, pride, rivalry, suspicion and other perfectly ordinary human passions.