ABSTRACT
The numerical simulation for the stability of disk galaxies without considering any extra mass distribution indicated that the disk galaxies would be deformed from their circular shapes and ultimately will be unstable. The numerical studies also pointed out that the rotational disk not only becomes globally unstable but also would rapidly evolve toward a pressure supported system. But this contradicts the observational reality such as our own Milky Way, which is a stable rotationally supported disk galaxy. This called for the existence of additional distribution of mass around the galaxy. The other observational and phenomenological analyses such as Zwicky’s observation of the Coma cluster or the behavior of rotational velocities for spiral galaxies clearly point to the existence of huge amounts of unseen matter or dark matter in galaxies and also in galaxy clusters. The need to explain the velocity of approach of the Milky Way toward the nearby galaxy Andromeda (or M31 or NGC 224) also requires the presence of excess mass in the galaxy.
