The Hubble Space Telescope has been used to find much of the Universe's missing hydrogen.

For many years, astronomers have been hunting for the vast quantities of hydrogen that were thought to have been "cooked up" in the Big Bang but which have somehow managed to disappear into the empty blackness of space.

Now they think they have identified it - ironically by finding oxygen.

The HST used the light of a distant quasar - the core of an active galaxy - to probe the intervening space.

The telescope's Imaging Spectrograph detected the spectral "fingerprints" of highly ionised oxygen superimposed on the quasar's light. Oxygen is a tracer for the hydrogen.

Fundamental models

Astronomers believe at least 90% of the matter in the Universe is hidden from view in an exotic "dark" form that has not yet been seen directly.

But more embarrassing is that, until now, they have not been able to see most of the Universe's ordinary matter either. The ordinary, or baryonic, matter is made of normal protons, electrons and neutrons.

Hubble's latest discovery will shed new light on the large-scale structure of the Universe.

The detection also confirms fundamental models of how much hydrogen was manufactured in the earliest stages of the Universe's birth.

"This is a successful, fundamental test of cosmological models," said Todd Tripp of Princeton University, New Jersey, US. "This provides strong evidence that the models are on the right track."

Dr Tripp, and Blair Savage from the University of Wisconsin at Madison, together with colleagues, report their research in the Astrophysical Journal Letters.

Supercomputer models

They describe how they used the light of a distant quasar to peer through the invisible space between galaxies, like shining a flashlight beam through a fog.

They could not look for the hydrogen directly because the element is fully ionised, which means the atoms are stripped of their electrons. Without electrons, no spectral features are etched into the quasar's Earth-bound light.

The oxygen on the other hand, although also highly ionised, still retains a few electrons which absorb specific colours from the quasar's light.

They say the oxygen tracer was probably created when exploding stars spewed the oxygen out into the space between galaxies. There it would have mixed with hydrogen and been heated to very high temperatures.

Supercomputer models of the expanding and evolving Universe show a web of gas filaments, with hydrogen, the simplest and lightest element, concentrated along vast chain-like structures. Clusters of galaxies form where the filaments intersect.

The models predict that vast hydrogen clouds flowing along the chains should collide and heat up, stopping the formation of galaxies in the hottest regions. That would mean stars were more commonly formed in the early Universe when the hydrogen was cool enough to coalesce.