A
dark matter halo is a
hypothetical component of a
galaxy that envelops the
galactic disc and extends well beyond the edge of the visible galaxy. The halo's
mass dominates the total mass. Since they consist of
dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of
stars and gas in galaxies. Dark matter halos play a key role in current models of
galaxy formation and evolution.