In recent years, astronomers have come to know more about the makeup of our Universe: 70% dark energy, 25% dark matter (both equally mysterious) and about 5% ordinary matter.

 According to the standard cosmological model, the total number of elementary particles that form this ordinary matter (baryons like protons and neutrons) has remained constant since
Big Bang. However, the baryons detected in our near Universe are half as numerous as those in the Big Bang Universe. To account for the missing half, the theory therefore predicts the existence of what is called the WHIM (Warm-Hot Intergalactic Medium), an intergalactic web of hot and diffuse gases. Continuing work published two years ago by four teams of astronomers, Fabrizio Nicastro, of the Harvard-Smithsonian Center for Astrophysics, and his colleagues studied the absorption spectrum of quasar Markarian 421, using data from the Observatory Chandra X-ray and ultraviolet emissions observations. they have
thus discovered the presence of ions (carbon, nitrogen, oxygen and neon) in two clouds of gas heated to nearly a million degrees Celsius crossed by the quasar. By extrapolating the size of these WHIM representatives located 150 and 370 million light years from Earth to the entire Universe, scientists were able to accurately estimate the density of baryons contained in this type of medium. .

 And this estimate corresponds to the missing mass. New instruments will undoubtedly be necessary to finalize this research. It was planned to install a spectrograph on Hubble
but the uncertain future of the telescope now compromises this project.

NYT 08 / 02 / 05 (Recovering lost atoms of cosmos)
http://www.nytimes.com/2005/02/08/science/space/08mass.html
http://chandra.harvard.edu/press/05_releases/press_020205.html
http://web.mit.edu/newsoffice/2002/hotgas-0814.html