Full story here.
The cloud, called a halo, appears to be enormous, extending hundreds of thousands of light-years across. Scientists suspect it is composed mainly of hydrogen, with some oxygen and other elements. The halo's temperature, size and mass were estimated using data from NASA's Chandra X-ray Observatory, the European Space Agency's XMM-Newton space observatory and Japan's Suzaku satellite.
Researchers think the mass inside this halo could be the answer to what's called the "missing baryon problem." Baryons are a class of subatomic particles that includes the protons and neutrons that make up the atoms inside stars and galaxies.
Theories of the formation and evolution of the universe predict there should be many more baryons than we see. In fact, the baryons that have been accounted for in our local cosmic neighborhood are only half of those predicted to exist there.
Galaxy-shrouding gas haloes, such as the one around the Milky Way, may be the hiding spot for many of these missing baryons.
"Although there are uncertainties, the work by Gupta and colleagues provides the best evidence yet that the galaxy's missing baryons have been hiding in a halo of million-Kelvin gas that envelopes the galaxy," NASA officials wrote in a statement. "The estimated density of this halo is so low that similar halos around other galaxies would have escaped detection."