The phrase "galaxies are cannibals" is metaphorical and refers to the phenomenon where galaxies merge or collide with each other, effectively "consuming" one another in a cosmic game of survival and evolution. When two or more galaxies come close enough, their gravitational forces start interacting and can eventually lead to a merger if the forces between them are strong enough.

As galaxies merge, stars, gas, and dark matter from one galaxy are incorporated into the other. During this process, the structures of the galaxies involved can be significantly disturbed, and new star formation can be triggered. The outcome of a galactic collision or merger can take various forms, including elliptical, spiral, or irregular galaxies, depending on the initial properties of the merging systems.

The idea that galaxies are cannibals arose from observations and studies in astronomy suggesting that many galaxies show evidence of interactions or mergers with other galaxies in their past. Astronomers use various techniques, such as studying galaxy shapes, detecting tidal tails, and measuring star formation rates, to identify galaxies undergoing or that have undergone mergers.

Galactic cannibalism is an important aspect of galaxy evolution. It plays a role in shaping the structures, properties, and distribution of galaxies in the universe and drives processes such as star formation and the accumulation of mass in galaxies. Understanding galaxy interactions and mergers helps astronomers study the formation and evolution of galaxies over cosmic time and gain insight into the dynamics and physics governing these massive structures.