In Star Trek, a galactic cluster is a grouping of stars formed from the same molecular cloud and bound together through gravitational attraction. The Borg used this term for designating certain regions of space. For example, in the Star Trek: Voyager episode "The Gift", Galactic Cluster 3 is the Borg designation for a transmaterial energy plane intersecting 22 billion omnicordial lifeforms, including Species 259.
There are, in fact, galactic glusters in the real universe. Galactic clusters (more commonly called open clusters) are groups of up to a few thousand stars that were formed from the same giant molecular cloud, and are still gravitationally (though quite loosely and weakly) bound to each other.
Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring. They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center, as well as losing cluster members through internal close encounters. As opposed to open clusters, where the stars are held together only weakly by gravity, globular clusters are very tightly bound by gravity.
In addition to galactic clusters within galaxies, there are also clusters of galaxies. These may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger groups called superclusters. Notable galaxy clusters in the relatively nearby universe include the Virgo cluster, Hercules Cluster, and the Coma Cluster. A very large aggregation of galaxies known as the Great Attractor, dominated by the Norma cluster, is massive enough to affect the local expansion of the universe (Hubble flow).
The Hercules Cluster