Starting from the number of stars' and galaxies' revolutions (on the cluster level), it is possible to measure the mass of the dark matter, and to deduce its distribution. A great quantity of this matter should be within the galaxies, not in the galactic disc but in the form of a halo including the galaxy. Indeed, this configuration allows a stability of the galactic disc. Moreover, certain galaxies have rings perpendicular to the disc and composed of gas, dust and stars. There, the halo of matter would explain the formation and stability required. On the other hand, it is impossible that the dark matter be in the galactic disc, due to the fact that we shoudl then observe an oscillation perpendicular to the disc in the stars movement (which is not the case).
The study of satellite galaxies (small galaxies turning around other galaxies) obliges to think of very wide halos: approximately 200 or 300 kpc. By comparison, the Sun is located at approximately 8,6 kpc center of our galaxy. The galaxy of Andromède - galaxy nearest to us - is located at 725 kpc, that is to say a little more of the double of the halo radius of our galaxy. These halos could be common between close galaxies.