Well, I have to add that it is also a matter of the horrendously awful use of the word "halo" that has come to be standard usage by the astronomical community. If one takes the typical meaning of that word, i.e.,
Originally Posted by Geo Kaplan
"a geometric shape, usually in the form of a disk, circle, ring, or rayed structure, traditionally representing a radiant light around or above the head of a divine or sacred personage, or as to Meteorology, any of a variety of bright circles or arcs centered on the sun or moon, caused by the refraction or reflection of light by ice crystals, or an undesirable bright or dark ring surrounding an image on the fluorescent screen of a television tube,"
then it is understandable that the astronomical meaning can become easily misunderstood. Even Dictionary.com doesn't define the astronomical meaning very well: "a spherical cloud of gas clusters and stars that form part of a spiral galaxy." In fact, that's horrible! At least they got the spherical right. The dark matter halo is spherical in shape, typically centered on the galaxy center (galaxy collisions can alter this). Forrest has apparently forgotten Grey's excellent account of the dark matter distribution within that generally spherical shape, which is slightly tricky because it appears to interact only gravitationally, but otherwise dark matter "particles," whatever they are, do not interact with normal matter, and they don't even interact among themselves.