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Dynamical friction (or Chandrasekhar friction) is drag on an object passing between other objects because of the gravitational attraction. An object moving relative to a bunch of others draws them toward its path, decreasing the distance and thus increasing the gravitational attraction between the object and these others, and the object is slowed, momentum being conserved by the others being dragged along the same direction. The dragged bunch, on balance, end up with a bit more velocity toward the path of the object's travel. (Running this scenario in reverse, this surrounding bunch would begin with a bit of velocity away from the object's path so their distance from it increases as the object is nearing and passing them, which means they have less gravitational attraction to the object after it has passed.)
Dynamical friction is theorized to affect galaxy collisions, due to its effect on the constituent stars, making a galaxy merger more likely. Dark matter dynamical friction (slowing in this manner by dark matter, occasionally abbreviated DMDF) is thought to affect compact object mergers, and also satellite galaxies and globular clusters, making them more likely to merge with their host galaxy. If the fuzzy dark matter model is valid, its characteristics could significantly affect the timescale of such infalls.
Dynamical friction is also considered a factor in planet formation, a mechanism by which protoplanetary-disk material gradually slows embedded protoplanets, causing an inward radial drift: (unlike a protoplanet, the material orbits slower than Keplerian due to gas pressure).