Milky Way Halo
The outer "halo" region of the Milky Way, as with other galaxies, contains the vast majority of the mass of the Galaxy. Yet the exact mass and shape of the halo are not known. Unlike the Galaxy's spiral arms, which contain bright stars, the halo is mostly dark. The few stars in the halo are the oldest stars in the Galaxy, and include the globular clusters, but they can account for only a small fraction of the halo's mass. Understanding the exact nature of the Milky Way halo may help to explain the mysterious dark matter, which seems to pervade the universe.
Warren Brown, Nelson Caldwell, David Latham, Guillermo Torres