Half the universe's missing matter is found

The mystery of where a good chunk of space's normal matter has been hiding looks to have been solved

Scientists say they have been able to account for a missing percentage of normal matter, finally solving a long-running mystery which has long baffled observers. According to the latest research, half of the normal matter – which is made up of subatomic particles called baryons – exists in the form of filaments of gas that link the galaxies together. Since it is very weak and not hot enough, they say it has managed to evade detection by X-ray telescopes.

Normal matter has long been thought to make up five per cent of the cosmos, with dark matter and dark energy believed to account for the rest. But, until now, only half of it has been easily observable, flying in the face of scientific calculations. Astronomers could see just 10 per cent of normal baryonic matter in stars and nebulae, and a further 40 per cent in diffuse clouds within galaxies. Data from the ESA's Planck satellite, however, has allowed scientists to make a breakthrough.

Two teams, from the Institute of Space Astrophysics in France, and the University of Edinburgh, looked for a thermal signal called the Sunyaev Zel'dovich effect. It allows for the detection of very faint objects while looking for photons from the Cosmic Microwave Background (the afterglow of the Big Bang) as it travels through hot gas. By stacking the signals for the areas between pairs of galaxies that were believed to be connected by a strand of baryons (that include protons and neutrons), they found evidence of dense gas filaments.

"This result establishes the presence of ionised gas in largescale filaments, and suggests that the missing baryons problem may be resolved via observations of the cosmic web," says study researcher Anna de Graaff, from the University of Edinburgh.