Scientists have a good estimate for the mass and concentration of ordinary matter in the universe following the Big Bang. Sometime later, roughly half of this material apparently vanished in space.

"An inventory of all the baryons in stars and gas inside and outside of galaxies accounts for just over half the baryons that existed shortly after the Big Bang," explained Fabrizio Nicastro of the Harvard-Smithsonian Center for Astrophysics.

Baryons are a class of particles that includes protons and neutrons -- components that contribute to an atom's mass. The material is generally referred to as ordinary matter to distinguish it from potent but mysterious particles known as dark matter.

Nicastro and his research team used computer simulations created by astrophysicists to identify probable locations for the missing particles.

"Now we have found the likely hiding place of the missing baryons," said Nicastro. The simulations suggested that baryons might be found in the form of a wispy web of clouds characterized by very low densities, and temperatures from a few hundred thousand to one million degrees Celsius.

Called "Warm-Hot Intergalactic Matter," or WHIM, such clouds can be found around our Milky Way galaxy but have never before been discovered in deep space.

While observing the quasar Markarian 421, a galaxy with a supermassive black hole at its center, Chandra astronomers detected two possible WHIM gas clouds.

Changes made to the quasar's X-ray light filtering through the gas revealed the presence of the clouds. Further analysis of these clouds showed them to be one million degrees Celsius in temperature and composed of elements such as carbon, nitrogen and oxygen.

The clouds also possessed the telltale low density. All of these factors matched the computer model's formula for a WHIM cloud.

Chandra scientists are confident clouds like those along the path to Markarian 421 exist across the universe. Researchers also suspect they may be of comparable size and spacing. If this is true, the cumulative mass of particles forming the tangle of clouds would equal the total mass of the missing baryon matter.

The baryon matter that was once lost appears to have been found, located by researchers in what might best be called "Chandra's Web."

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