A dark matter halo is the dark matter portion of a galaxy which extends throughout the visible galaxy and further out, thus as a "halo". Dark matter explains apparent additional gravity of a galaxy, galaxy cluster, or other large structures beyond that explained by the visible and inferred stars, interstellar media and other normal matter. The amount of gravity is inferred by the velocity of the orbiting stars, determined by examining Doppler shifts of the starlight.
The typical inferred halo is spherical, densest in the center and about 20 times more massive than the visible portion of the galaxy.
Dark Matter Halo can also refer to a region of dark matter throughout a galaxy cluster, presumably somewhat less dense than regions around the individual galaxies. The term cluster dark matter halo is sometimes used to distinguish this kind.