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, Galactic Cluster 3 is the Borg designation for a transmaterial energy plane intersecting 22 billion omnicordial lifeforms. Species 259 was native to this region, prior to their assimilation. (VOY: "The Gift").

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.

Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest away.

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. Galaxy groups and clusters are the largest gravitationally-bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large scale structure of the universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger groups called superclusters.

When observed visually, clusters appear to be collections of galaxies held together by mutual gravitational attraction. However, their velocities are too large for them to remain gravitationally bound by their mutual attractions, implying the presence of either an additional invisible mass component, or an additional attractive force besides gravity. X-ray studies have revealed the presence of large amounts of intergalactic gas known as the intracluster medium. This gas is very hot, between 107K and 108K, and hence emits X-rays in the form of bremsstrahlung and atomic line emission. The total mass of the gas is greater than that of the galaxies by roughly a factor of two. However this is still not enough mass to keep the galaxies in the cluster. Since this gas is in approximate hydrostatic equilibrium with the overall cluster gravitational field, the total mass distribution can be determined. It turns out the total mass deduced from this measurement is approximately six times larger than the mass of the galaxies or the hot gas. The missing component is known as dark matter and its nature is unknown. In a typical cluster perhaps only 5% of the total mass is in the form of galaxies, maybe 10% in the form of hot X-ray emitting gas and the remainder is dark matter.

Clusters typically have the following properties.

* They contain 50 to 1000 galaxies, hot X-ray emitting gas and large amounts of dark matter
* The distribution of these three components is approximately the same in the cluster.
* They have total masses of 1014 to 1015 solar masses.
* They typically have a diameter from 2 to 10 Mpc *
The spread of velocities for the individual galaxies is about 800-1000 km/s.

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).