Accretion is defined as: matter that makes up galaxies. gravity that pushes matter in a circular manner into an outer source. gravity that pulls matter in a spiraling matter. none of the above

Accretion is defined as: gravity that pulls matter in a spiraling matter. 
This often used to explain the origin of planet or stars. In the space, particles will be lumped out toward one another due to some sort of gravitational pull that accumulates those particles into massive astronomical objects.