“Missing” matter could reside in hot galactic haloes

Ars Technica : Normal matterâmdash;the electrons and nuclei and other particles with which we are familiarâmdash;makes up only 20% of the universe’s total matter. However, a large portion of that normal matter is “missing,” meaning it does not appear to be part of celestial bodies that we can observe. The most popular models for explaining that missing matter propose that it exists in the form of extremely hot, low-density plasmaâmdash;either in the gravitational wells of galaxies or in the intergalactic medium. Although some evidence points to the latter location, searches for the matter in the former have been unsuccessful. Now, however, an international group of researchers has detected absorption lines from highly ionized oxygen when they used the Chandra X-Ray Observatory to look at distant quasars through the outskirts of the Milky Way. Although the researchers could not determine the exact size of the cloud, they were able to determine its density. Combining that value with the assumption that the cloud was roughly spherical, they calculated the cloud’s mass as being 12 billion times the mass of the Sun, which is comparable to the mass of the known matter in the Milky Way.