Globular clusters are dense, spherical concentrations of stars. They tend to reside in the halos of galaxies. The Milky Way is surrounded by a halo that is populated by globular clusters and satellite galaxies. A study by Zaritsky et al. (2016) suggests that some of these globular clusters actually belong inside the halos of undetected satellite galaxies of the Milky Way. This is supported by the recent confirmation of a globular cluster that is associated with the ultrafaint galaxy Eridanus II, a satellite galaxy of the Milky Way. The high surface brightness of a globular cluster due to its high concentration of stars can make it much easier to identify than its host galaxy. This is especially true if the host galaxy is strongly dominated by dark matter and has an extremely tiny population of stars scattered over a large area. As a result, some of the globular clusters in the halo of the Milky Way galaxy could be residing within the halos of undetected dark satellite galaxies.
Reference:
Zaritsky et al. (2016), "Are Some Milky Way Globular Clusters Hosted by Undiscovered Galaxies", arXiv:1604.08594 [astro-ph.GA]
