A "Matrioshka Brain" is a highly engineered solar system [1]. It is effectively a multi-layer Dyson Shell spanning solar system sized (and somewhat greater) volumes. They would be constructed by civilizations not too much more developed than we are that choose to stop wasting their solar energy. They could take a star "dark" in a short period of time (depending on the system less than a year to a few hundred years). A recent preprint discusses the fact that one logical place for a Matrioshka Brain to migrate is away from the galaxy (to compute more efficiently and reduce their hazard function) [2]. So yes it is completely feasible for there to be "dark" matter (though not of the type most physicists consider) to be orbiting around galaxies.

1. http://www.aeiveos.com/~bradbury/MatrioshkaBrains/index.html
2. http://arxiv.org/abs/astro-ph/0506110