A matter of fact: University scientists account for universe's missing matter

Ordinary matter — the same stuff that makes up a bag of Doritos, your English preceptor or a galaxy — is not always easy to see.

Until only a few weeks ago, scientists could view only half of the universe's ordinary — or baryonic — matter.

Baryonic matter, which is made up of protons, neutrons and electrons, accounts for 10 percent of the universe's mass. Scientists believe the other 90 percent is "dark matter," or a kind of matter that is not present on Earth.

Ray Villard, spokesman for the Space Telescope Science Institute, explained stars and galaxies make up about half of the universe's estimated baryonic matter. Models of the universe indicate more baryonic matter must be present to account for the observed behavior of the universe, though previously scientists did not know where that matter existed.

On May 1, researchers at Princeton and the University of Wisconsin published results that could account for most of the missing matter.

Astrophysics postdoctoral fellow Todd Tripp and astrophysics professor Edward Jenkins used the Hubble Space Telescope to find large amounts of hydrogen lying invisible between galaxies.

"We had indications that the universe contains a substantial amount of matter that is hot and tenuous," Tripp said, referring to supercomputer models of the universe that have predicted highly ionized hydrogen.

Hydrogen, at 100,000 degrees Kelvin, enters a plasma state and loses its electrons. At this temperature, the Hubble Space Telescope is unable to detect the hydrogen.

The telescope can, however, see the oxygen that accompanies the hot hydrogen.

"We thought a nice way to look for this stuff would be to use a signature from highly ionized oxygen," Tripp explained. "Whenever you see oxygen, you know a huge amount of hydrogen goes with it."

Jenkins commented on the significance of the discovery. "This is quite noteworthy," he said. "[The hydrogen] is a significant contribution to the amount of ordinary matter."

Villard explained the shape of the elusive hydrogen. "It seems like filaments in a sponge," he said. "The holes in the sponge are voids in space."

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Though Villard also said the hydrogen filaments probably make up most of the missing baryonic matter in the universe, the exact quantity of the hydrogen detected is still uncertain.

"This is exciting, but we need to tie up some loose ends," Tripp said. "We need to put our feet on solid ground."

Tripp and his collaborators will use their new findings to better understand the universe as they study the supercomputer models that predicted the hydrogen filaments.

"Basically our job is to test the theory of the superconductor models," Tripp said. "The scientists [who made the models] started at day one, put in all the physics they could think of and they let the computer go until the present day.

"The computer ended up working it out, and our observation has shown that the models are on the right track," he said.