What constitutes a galactic cluster?

A galactic cluster is characterized as a gravitationally bound group of galaxies. This type of structure is significant in the universe, as it can contain anywhere from a few dozen to thousands of galaxies that are held together by their mutual gravitational attraction. In these clusters, galaxies can interact with one another, leading to phenomena such as galaxy mergers and the development of larger cosmic structures.

Galactic clusters play a crucial role in the understanding of cosmological evolution and the distribution of matter in the universe. They often harbor a wealth of information about dark matter, as the visible matter (the galaxies) does not account for the total mass inferred from gravitational effects observed in the cluster, leading to the conclusion that a significant amount of matter is not directly visible.

The other options, while related to astronomical phenomena, do not accurately define what a galactic cluster is. For instance, a group of stars tightly bound by gravity refers more to star clusters or globular clusters, while a collection of stars within a nebula pertains to regions of star formation. Similarly, a collection of meteorites in space does not relate to the organization of galaxies and therefore does not fit the definition of a galactic cluster.