In the halo, the stars are packed in globular clusters, which hold higher or lower metallicities according to their position in regard to the galactic centre (those located closer to the centre hold lower metallicities and those located closer to it hold higher metallicities).
The globular clusters can display very high densities (like 1 million stars tightly packed in a sphere only 30 light years wide) or can be very rarefied. They are composed by a range between 50 000 to several million stars located around a nucleus and have a vaguely spherical distribution.
It's estimated that the globular clusters that are most distant from the galactic centre have been formed during the epoch when occurred the collapse of the gaseous mass that generated the primordial body of the Milky Way. On the other hand, those who are closer to the centre would have initially been dwarf galaxies that were later captured by our galaxy.
The movement of the globular clusters, cyclically crossing the plane of the disk, would have stripped the clusters from the early gas content, so none of it was left in order to forge new stars. It's possibly for that reason that the stars of the globular clusters are the oldest known.
A globular cluster (Davidson E. Soper)
Dark Matter and Interstellar Material
Given the high rotation speed of the outer layers of the spiral galaxies, it's thought that, along with the globular clusters, the halo concentrates huge amounts of dark matter, which still has an uncertain nature, and some other rarefied interstellar material mainly composed by protons and electrons.