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.