A disc is a component of disc galaxies, such as spiral galaxies, or lenticular galaxies.

The galactic disc is the plane in which the spirals, bars and discs of disc galaxies exist. Galaxy discs tend to have more gas and dust, and younger stars than galactic bulges, or galactic haloes.

The galactic disc is mainly composed of gas, dust and stars. The gas and dust component of the galactic disk is called the gaseous disk. The star component of the galactic disk is called the stellar disk.

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

Dark matter

In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

.