The orbits of objects such as globular clusters and satellite galaxies carry them through the extended dark matter halo (dark halo) and sometimes the dense central regions of their host galaxies. As a satellite moves through this 'sea' of stars and dark matter, it experiences a drag which slows it down. This drag is referred to as dynamical friction.
The drag results from the satellite accelerating stars and dark matter in its wake - thus increasing their energy and momentum. As a consequence of the conservation of energy and momentum, the satellite itself must therefore lose energy/momentum and slow down. Over time, this causes the satellite to spiral in towards the centre of the host galaxy. This is one of the main driving mechanisms for minor mergers.
The dynamical friction experienced by a satellite depends on:
Galaxies moving through the extended dark matter halos of groups and clusters also experience dynamical friction, albeit at much lower levels than that experienced by objects orbiting within galaxies.