Astronomers using the Hubble Space Telescope have gathered the first observational evidence of a vast "web" of hydrogen gas that theoretical models predict stretches across the dark void between galaxies.

The hydrogen takes the form of enormous, chain-like structures, or filaments -- "sort of like an extremely complicated and messy spider web," said Princeton University astrophysicist Todd Tripp, a member of the team that made the observation.

Such hydrogen is believed to make up nearly half of the universe's "normal" matter which is composed of familiar protons, neutrons and electrons and that was, up until now, elusive to astronomers. (The missing hydrogen should not be confused with the exotic "dark matter" believed to comprise over 90 percent of the mass in the universe.)

The Hubble Space Telescope gathered evidence of an intergalactic web of hydrogen by analyzing light from a distant quasar.

The hydrogen could not be observed directly, since it is ionized, or stripped of electrons (and thus does not absorb light from the quasar). The oxygen is also highly ionized, but still contains a few electrons. The gases are believed to have become heated and ionized due to shock waves from galactic collisions.

The observing team included Tripp, Edward Jenkins of Princeton and Blair Savage of the University of Wisconsin at Madison. The results are published in the May 1 issue of Astrophysical Journal Letters.

Tripp noted that the conclusion is preliminary and said further observations, including those of X-ray telescopes, would be needed to confirm there's an intergalactic hydrogen web. "This is the first indication that we're not barking up the wrong tree, that we're on to something interesting," he said.

The arrow in this image, taken by a ground-based telescope, points to the quasar whose light was analyzed by the team using Hubble.