dynamical friction
A heavy ship moving through water leaves a wake that drags on it. A massive object moving through a sea of stars does something eerily similar — but with gravity instead of water. As it plows along, it gathers a trailing crowd of stars pulled in behind it, and the gravity of that overdense wake tugs it backward, gently but steadily slowing it down. This gravitational drag is called dynamical friction.
There is no actual friction or contact — only the cumulative gravitational pull of the wake of stars that the heavy body deflects toward itself. Because the wake forms behind the moving mass, its net tug points backward along the motion, draining the heavy object's orbital energy. The effect is stronger for more massive intruders and in denser surroundings, and it transfers the lost energy into the random motions of the background stars, heating them up. First worked out by Chandrasekhar in 1943, it is friction made entirely of gravity.
Dynamical friction quietly governs the long-term life of galaxies and clusters. It causes massive star clusters and satellite galaxies to spiral inward toward a galaxy's center over billions of years, and it is the key process by which two supermassive black holes, after their host galaxies merge, are dragged together close enough to eventually spiral in and coalesce. It is also why heavy stars in a cluster tend to sink toward the middle while lighter ones drift outward — gravity sorting a crowd by mass.
When a small galaxy falls into a larger one, dynamical friction drags it relentlessly toward the center, where it is shredded and absorbed — one of the slow engines of how big galaxies grow by swallowing smaller ones.
Gravitational drag is how merging galaxies bring their cores together.
Despite the name there is no contact or heat-of-rubbing; it is purely the back-reaction of the gravitational wake. The lost orbital energy is not destroyed but passed into the random motions of the surrounding stars.