A Tour of Where is the Universe Hiding its Missing Mass?

Narrator (April Jubett, CXC): Astronomers have spent decades looking for something that sounds like it would be hard to miss: about a third of the "normal" matter in the Universe. New results from NASA's Chandra X-ray Observatory may have helped them locate this elusive expanse of missing matter.

From independent, well-established observations, scientists have confidently calculated how much normal matter — meaning hydrogen, helium and other elements — existed soon after the Big Bang. In the time between the first few minutes and the first billion years or so, much of the normal matter made its way into gas and objects such as stars and planets, observed in the present-day Universe.

The problem is that when astronomers add up the mass of all the normal matter in the present-day Universe about a third of it can't be found. (This missing matter is distinct from the still-mysterious dark matter.)

One idea is that the missing mass gathered into gigantic strands or filaments of warm (that is, temperature less than 100,000 Kelvin) gas and hot (as in hotter than 100,000 Kelvin) gas in intergalactic space. These filaments are known by astronomers as the "warm-hot intergalactic medium" or WHIM. They are invisible to optical light telescopes, but some of the warm gas in filaments has been detected in ultraviolet light.

Using a new technique, researchers have found new and strong evidence for the hot component of the WHIM based on data from Chandra and other telescopes. They used Chandra to look for and study filaments of warm gas lying along the path to a quasar, a bright source of X-rays powered by a rapidly growing supermassive black hole. This quasar is located about 3.4 billion light years from Earth.

Their work revealed an absorption line from oxygen expected to be present in a gas with a temperature of about one million Kelvin. By extrapolating from these observations of oxygen to the full set of elements, and from the observed region to the local Universe, the researchers report they can account for the complete amount of missing matter.

If this result is confirmed, one of the biggest puzzles in modern astrophysics could be solved.