Friction in this context refers to "dynamical friction", where massive objects -- e.g., dense cluster cores -- moving through a medium of many small masses -- e

Friction in this context refers to "dynamical friction", where massive objects -- e.g., dense cluster cores -- moving through a medium of many small masses -- e.g., DM particles -- will experience gravitational drag and slow down. https://en.wikipedia.org/wiki/Dynamical_friction https://en.wikipedia.org/wiki/Dynamical_friction