The galactic disc is mainly composed of gas, dust and stars. The gas and dust component of the galactic disc is called the gaseous disc. The star component of the galactic disc is called the stellar disc.
Inconsistent orbital velocities of stars
It has been noted that the orbital velocity of stars in the disc of most disc galaxies is inconsistent with the amount of luminous matter calculated for the galaxy. A possible explanation for this problem is the non-luminous dark matter.