New X-ray observations add further evidence to the likelihood that most of the universe is comprised of exotic dark matter.  The finding may also help narrow down the types of dark matter researchers should consider viable.
    Most astronomers already view dark matter as the only logical way to explain and scientists don’t know exactly what it is, but without it galaxies would fly apart.
     Still, a competing theory suggests the universe contains plenty of regular matter but that its effects at the outskirts of a galaxy are less than what most scientists predict.
    The new data, from NASA (news - web sites)'s Chandra X-ray Observatory,   shows a galaxy called NGC 720 is enveloped in a slightly flattened, or ellipsoidal cloud of hot gas that has an orientation different from that of the optical image of the galaxy. Hot gas emits X-rays but cannot be detected in optical surveys.
    "The shape and orientation of the hot gas cloud require it to be confined by an egg-shaped dark matter halo," said David Buote of the University of California, Irvine, and lead author of a report on this research in the Sept. 20 issue of The Astrophysical Journal. "This means that dark matter is not just an illusion due to a shortcoming of the standard theory of gravity -- it is real."
    Without the dark matter, the hot gas observed by Chandra would expand away, the thinking goes.  Buote and his colleagues also found that the Chandra data fit predictions of the cold dark matter theories, according to which dark matter consists of slowly moving particles, which interact with each other and "normal" matter only through gravity. Other forms of theorized dark matter, such as self-interacting dark matter and cold molecular dark matter, are not consistent with the observation in that they require a dark matter halo that is too round or too flat, respectively. The conclusion assumes that the hot gas cloud has not been overly disturbed by collisions or mergers with other galaxies in the last 100 million years.  NGC 720 is about 80 million light years from Earth.