It is one of cosmology’s more perplexing problems: that up to 90% of the ordinary matter in the universe appears to have gone missing.
Now astronomers have detected about half of this missing content for the first time, in a discovery that could resolve a long-standing paradox.
The conundrum first arose from measurements of radiation left over from the Big Bang, which allowed scientists to calculate how much matter there is in the universe and what form it takes. This showed that about 5% of the mass in the universe comes in the form of ordinary matter, with the rest being accounted for by dark matter and dark energy.
Dark matter has never been directly observed and the nature of dark energy is almost completely mysterious, but even tracking down the 5% of ordinary stuff has proved more complicated than expected. When scientists have counted up all the observable objects in the sky – stars, planets, galaxies and so on – this only seems to account for between a 10th and a fifth of what ought to be out there.
The deficit is known as the “missing baryon problem”, baryons being ordinary subatomic particles like protons and neutrons.
Richard Ellis, a professor of astrophysics at the University College London, said: “People agree that there’s a lot missing, raising the question where is it?”
The distribution of galaxies in the universe follows a web-like pattern and scientists have speculated that the missing baryons could be floating in diffuse gaseous filaments and sheets linking the galaxy clusters in the cosmic web.
Theoretical calculations suggest these gaseous threads, known as the warm–hot intergalactic medium, or the Whim, ought to be around a million degrees celsius. A mist of gas at this temperature is too cold to emit X-rays that could be spotted by ordinary telescopes from the Earth – but not cold enough to absorb significant amounts of light passing through it.
“The trouble is, it’s in this unusual temperature regime where we…