Astronomers have argued about high-velocity clouds for over thirty-five years. Wakker and colleagues detected ionized sulphur in one such cloud using the Hubble Space Telescope. According to their calculations, the abundance of heavy elements in the cloud is only 9% of that found in the Sun. This, says Wakker, strongly suggests that the clouds replenish galaxies with hydrogen.

The discovery of molecular hydrogen by Richter's team disputes this argument. Molecular hydrogen is usually created on dust grains composed of heavy elements such as iron. However, these grains could be debris from the nearby Magellanic Clouds - a set of 'mini' galaxies orbiting the Milky Way. This implies that the clouds studied by Richter's team are not typical of high-velocity clouds in general.

The only answer is to make more measurements from other clouds says Robert Braun of the Netherlands Foundation for Research in Astronomy.