It has been postulated,and generally accepted, that the dark matter would have to be located in a massive, roughly spherical halo enshrouding each galaxy. The first real surprise in the study of dark matter lay in the outermost parts of galaxies, known as galaxy halos. Here there is negligible luminosity, yet there are occasional orbiting gas clouds which allow one to measure rotation speeds and distances. The rotation speed is found not to decrease with increasing distance from the galactic center, implying that the mass distribution of the galaxy cannot be concentrated, like the light distribution. This rise appears to stop at about 50kpc, where halos appear to be truncated.
Many people believe that the galactic halos are composed of particle dark matter.
The Center Direct Detection Group is actively searching for evidence of this dark matter.