WASHINGTON. May 4 Astronomers using the' Hubble Space Telescope said on Wednesday they had found much of the universe's missing hydrogen, ironically by finding oxygen, reports Reuters.

One of the most embarrassing mysteries facing astronomers Is that they cannot find most of the matter in the universe. They know how much should be there by the behaviour of galaxies and other bodies.

But they cannot see it.

They believe about 90 per cent of the missing matter is called "dark matter? which is a special kind of matter not seen on Earth.

But a lot of the so-called baryonic matter — the regular stuff made up of protons, neutrons and electrons — also could not be accounted for. Astronomers think a lot of it is tied up in the form of highly ionised, or charged, hydrogen In between galaxies.

Todd Tripp of Princeton University. Blair Savage from the University of Wisconsin at Madison and colleagues used the orbiting Hubble Space Telescope to look for it — not by looking directly, because hot hydrogen is hard to see, but by looking for ionised oxygen.

Writing in astrophysical Journal Letters, they descried how they used the light of a distant quasar to pfeer through the invisible space between the galaxies, like shining a flash-Ught beam through a fog.

The Space Telescope Imaging Spectrograph aboard Hubble detected the spectral signature of oxygen superimposed on the quasar's light.

They said 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.

Supercomputer models of the expanding and evolving universe show a web of gas filaments. with hydrogen, the simplest and lightest element, concentrated along vast chainlike structures.

The models predict that vast hydrogen clouds flowing along Hie chains should collide and heat up. stopping the formation of galaxies in the hottest regions.