The motion of stars in galaxies and the motion of galaxies in clusters imply that there is about 10 fold more mass as in the luminous objects we can see. The indirectly observed non-luminous matter is...The motion of stars in galaxies and the motion of galaxies in clusters imply that there is about 10 fold more mass as in the luminous objects we can see. The indirectly observed non-luminous matter is called dark matter. Why is dark matter a problem? For one thing, we do not know what it is. It may well be 90% of all matter in the universe, yet there is a possibility that it is of a completely unknown form -- a stunning discovery if verified. Dark matter has implications for particle physics.
