Galaxies are vast collections of stars, gas, dust, and dark matter, organized by gravity. Our own Milky Way galaxy contains hundreds of billions of stars, but it is only one island in an ocean of other galaxies. These massive stellar systems are not scattered randomly; they are fundamental building blocks that arrange themselves into progressively larger, more complex structures. Understanding “what comes after a galaxy” requires expanding our view to the cosmic scales of their neighborhoods and the ultimate fate of the universe.
Galaxy Groups and Clusters
The immediate structure beyond an individual galaxy is a gravitationally bound collection of neighbors, known as a galaxy group. Our own galaxy is a member of the Local Group, which typically contains 50 or fewer member galaxies and spans about three million light-years. This aggregation is held together by the collective gravitational pull of its members and the surrounding dark matter halo.
Moving up in scale, a galaxy cluster represents a more massive and densely populated structure. Clusters contain hundreds to thousands of galaxies and can stretch for several million light-years. For instance, the Virgo Cluster forms the heart of our larger local region, containing over a thousand galaxies centered on a few giant elliptical galaxies.
These massive structures are dominated by a pervasive, extremely hot gas known as the intracluster medium (ICM). The ICM is heated to millions of degrees, emitting powerful X-rays that map the cluster’s shape. This hot plasma and dark matter account for up to 90% of the total mass. Together, they provide the gravitational scaffolding that prevents the fast-moving galaxies from flying apart.
Superclusters and the Cosmic Web
Beyond the gravitationally bound clusters are even larger formations called superclusters. A supercluster is essentially a cluster of galaxy clusters, where the individual groups and clusters are loosely linked by the remnants of the early universe’s density fluctuations. Our Local Group resides within the Laniakea Supercluster.
Superclusters are woven into the largest structure in the universe, known as the Cosmic Web. This web-like architecture results from gravity acting on matter over billions of years, creating a vast, sponge-like network. Galaxies and clusters congregate in long, thread-like structures called filaments or walls, which are the highest-density regions.
These filaments, which can be hundreds of millions of light-years long, surround and separate regions of space called Voids. Voids are areas largely devoid of galaxies, making up approximately 80% of the universe’s volume. Voids are the vast, under-dense bubbles that define the universe’s macroscopic structure.
The Long-Term Fate of Individual Galaxies
Galaxies undergo significant internal evolution, even while part of larger cosmic structures. One dramatic event is galaxy merging, driven by mutual gravitational attraction between nearby stellar systems. The Milky Way and the Andromeda galaxy are currently hurtling toward each other.
In about four to five billion years, these two spiral galaxies will begin a gravitational dance resulting in a single, larger elliptical galaxy, sometimes nicknamed “Milkomeda.” Direct stellar collisions are extremely rare because the distance between individual stars is vast. The primary result of the merger is a rearrangement of stellar orbits and a burst of star formation as gas clouds collide.
The merger process consumes or ejects the majority of the galaxy’s cold gas, which fuels new stars. This loss of star-forming material marks the end of the galaxy’s active life. It leads to the formation of a “red and dead” elliptical system containing mostly old, red stars. This transformation represents the final stage in the life cycle of a massive galaxy.
The Expanding Universe and Cosmic Endpoints
The ultimate fate of all cosmic structures is determined by the accelerating expansion of the universe. This expansion is driven by Dark Energy, a mysterious force accounting for about 68% of the total energy density. Dark Energy acts as a form of negative pressure, causing the fabric of space-time to stretch faster.
This accelerated expansion creates a Cosmic Horizon, pushing all but the most gravitationally bound structures out of sight. Galaxies outside our Local Group will recede so quickly that their light will never reach us, isolating our corner of the universe. Over about 100 billion years, the night sky will become almost completely dark, save for the few nearby galaxies that remain gravitationally bound.
The prevailing theory for the universe’s final state, assuming Dark Energy remains constant, is the Heat Death, or Big Freeze. In this scenario, all stars eventually burn out, matter decays, and black holes evaporate through Hawking radiation. The universe will continue to expand, becoming uniformly cold, dark, and dilute, reaching a state of maximum entropy where no further meaningful activity is possible.