Answer:
Dark matter hold the galaxy, give them spin and add to their mass.
Step-by-step explanation:
After the Big-bang theory, all the atoms and particles including dark matter was scattered in the Universe and massive amount of heat was released. After years when temperature dropped, dark matter collected all the particles, through this way universe was formed.
Hence dark matter gives mass to galaxy and provide spin as the effect of its gravity is observed by the movement of various celestial bodies.