Astronomers detected vast filaments of invisible hydrogen by using the light of a distant quasar (core of active galaxy) to probe the dark space between the galaxies. The Hubble Space Telescope Imaging Spectrograph found the spectral "fingerprints" of highly ionized intervening oxygen (which is a tracer of the hydrogen) superimposed on the quasar's light. Slicing across billions of light-years of space, the quasar's brilliant beam penetrated at least four separate filaments of the invisible hydrogen laced with the telltale oxygen. This filamentary structure is throughout the universe, all the way out to the distance of the quasar. For simplification, this graphic isolates the filamentary structure to a specific location along the line of sight to the quasar. Illustration: John Godfrey (STScI)

For the past decade astronomers have looked for vast quantities of hydrogen that were cooked-up in the Big Bang but somehow managed to disappear into the empty blackness of space.

Now, NASA's Hubble Space Telescope has uncovered this long-sought missing hydrogen. It accounts for nearly half of the "normal" matter in the universe -- the rest is locked up in myriad galaxies.

Astronomers believe at least 90 percent of the matter in the universe is hidden in 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, or baryonic, matter (normal protons, electrons and neutrons).

The confirmation of this missing hydrogen 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 first few minutes of the universe's birth in the Big Bang.

"This is a successful, fundamental test of cosmological models," said Todd Tripp of Princeton University, Princeton, NJ. "This provides strong evidence that the models are on the right track." The results of Tripp and his collaborators, Edward Jenkins from Princeton and Blair Savage from the University of Wisconsin-Madison, are being published in the May 1 issue of the Astrophysical Journal Letters.

Previous observations show that billions of years ago this missing matter formed vast complexes of hydrogen clouds -- but since then has vanished. Even Hubble's keen eye didn't see the hydrogen directly because it is too hot and rarified. Instead, Hubble found a telltale elemental tracer -- highly ionized (energized) oxygen -- between galaxies, which the hydrogen heats to the temperatures observed in intergalactic space. The presence of highly ionized oxygen between the galaxies implies there are huge quantities of hydrogen in the universe, which is so hot it escapes detection by normal observational techniques.

The arrow in this image, taken by a ground-based telescope, points to a distant quasar, the brilliant core of an active galaxy residing billions of light-years from Earth. As light from this faraway object travels across space, it picks up information on galaxies and the vast clouds of material between galaxies as it moves through them. The Space Telescope Imaging Spectrograph aboard NASA's Hubble Space Telescope decoded the quasar's light to find the spectral "fingerprints" of highly ionized (energized) oxygen, which had mixed with invisible clouds of hydrogen in intergalactic space. The quasar's brilliant beam pierced at least four separate filaments of the invisible hydrogen laced with the telltale oxygen. The presence of oxygen between the galaxies implies there are huge quantities of hydrogen in the universe. Photo: WIYN Telescope at Kitt Peak National Observatory, Arizona

In recent years, supercomputer models of the expanding, evolving universe have predicted an intricate web of gas filaments where hydrogen is 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. This would squelch the formation of more galaxies in the hottest regions, so star birth was more abundant in the early universe when the hydrogen was cool enough to coalesce.

The oxygen "tracer" was probably created when exploding stars in galaxies spewed the oxygen (created in their cores through nuclear fusion) back into intergalactic space where it mixed with the hydrogen and then was shocked and heated to temperatures over 360,000 degrees Fahrenheit (100,000 degrees Kelvin).

Astronomers detected the highly ionized oxygen by using the light of a distant quasar to probe the invisible space between the galaxies, like shining a flashlight beam through a fog. Hubble's Space Telescope Imaging Spectrograph found the spectral "fingerprints" of intervening oxygen superimposed on the quasar's light. Slicing across billions of light-years of space, the quasar's brilliant beam penetrated at least four separate filaments of the invisible hydrogen laced with the telltale oxygen.

Hubble's ultraviolet sensitivity and high-resolution spectroscopic capability allowed it to probe the nearby universe, where spectral features of hot gas can be seen at ultraviolet wavelengths and the problems faced by X-ray astronomers are avoided. "This result beautifully illustrates the power of spectroscopy for revealing fundamental information about the presence and nature of the gaseous matter in the universe," according to Hubble spectroscopist Blair Savage.

Still, the hot hydrogen could not be seen directly because it is fully ionized and so the hydrogen atoms are stripped of their electrons. Without electrons, no spectral features were etched into the quasar's earth-bound light. The oxygen is highly ionized too, but still retains a few electrons which absorb specific colors from the quasar's light.