A large, relatively dust-free, spherical region surrounding a spiral galaxy, such as our own. The inner, visible part of the halo, which has roughly the same diameter as the galactic disk, is occupied by Population II objects, including globular clusters and old, individual stars. Beyond this is a much larger region, called the dark halo or extended halo, containing large amounts of dark matter, the presence of which is revealed by its gravitational effect on the galaxy's rotation. The nature of galactic dark matter is still undetermined but the most popular theory, supported by some recent observations, is that the dark halo is home to vast numbers of small, unseen bodies known as MACHOs.
