A galaxy cluster is a massive collection of galaxies bound together by gravity, typically containing hundreds to thousands of individual galaxies. These are the largest structures in the universe held together by gravitational force, spanning anywhere from about 3 million to 33 million light-years across. But the galaxies themselves are only a small fraction of what makes up a cluster. Most of the mass is invisible dark matter, and the space between the galaxies is filled with superheated gas that outweighs all the stars combined.
How Big Galaxy Clusters Actually Are
Galaxy clusters range from roughly 1 to 10 megaparsecs in diameter, which translates to about 3.26 million to 32.6 million light-years. To put that in perspective, our entire Milky Way galaxy is about 100,000 light-years across. A large galaxy cluster could line up more than 300 Milky Ways side by side.
The mass contained in these structures is staggering. A typical cluster weighs in at around 100 trillion to 1,000 trillion times the mass of our Sun. Only about 2 to 5 percent of that mass is in the form of visible galaxies. Another 10 to 15 percent is extremely hot gas filling the space between galaxies. The remaining 80 percent or more is dark matter, a substance that doesn’t emit or absorb light but exerts gravitational pull on everything around it.
Groups vs. Clusters
Not every collection of galaxies qualifies as a cluster. Smaller gatherings are called galaxy groups, and the dividing line between the two is somewhat fuzzy. In practice, astronomers often use a mass threshold of around 100 trillion solar masses to separate clusters from groups. Groups may have only a handful of luminous galaxies (sometimes fewer than 10), while true clusters contain hundreds or thousands. Our own Milky Way belongs to the Local Group, a modest collection of about 80 galaxies that falls well short of cluster status.
What Fills the Space Between Galaxies
The vast emptiness between galaxies in a cluster isn’t actually empty. It’s filled with an extremely thin, extraordinarily hot gas called the intracluster medium. This gas reaches temperatures between 20 million and 100 million degrees Kelvin, hot enough that its atoms are completely stripped of their electrons. At those temperatures, the gas glows brightly in X-rays, which is how astronomers first detected it in the 1960s when X-ray observations of the Coma cluster revealed diffuse emission that couldn’t be explained by the galaxies alone.
The gas is incredibly sparse, with roughly one atom per thousand cubic centimeters, far more tenuous than any vacuum achievable on Earth. Yet there’s so much volume in a cluster that the total mass of this hot gas adds up to several times the mass of all the galaxies’ stars put together. This makes the intracluster medium the dominant form of ordinary (non-dark) matter in clusters.
How Galaxy Clusters Form
Galaxy clusters sit at the nodes of the cosmic web, a vast network of filaments, sheets, and voids that gives the universe its large-scale structure. This web is the result of gravity amplifying tiny density variations left over from the earliest moments after the Big Bang. Regions with slightly more matter pulled in surrounding material, growing denser over billions of years.
The process follows a clear flow pattern: matter drains out of voids into flat sheets, then funnels along filaments, and finally accumulates at the intersections where filaments meet. These intersections are where clusters form. Almost all the matter that ends up in a cluster arrives along filaments rather than from surrounding sheets or voids.
Clusters are latecomers in cosmic history. At early times, the universe was dominated by thin filaments and sheets. Cluster regions only began accumulating a significant share of the universe’s matter relatively recently in cosmic terms, within the last 5 to 8 billion years. Today’s clusters are the product of smaller groups and filaments merging together over time, building progressively larger structures.
Notable Galaxy Clusters
The Virgo Cluster is the nearest large cluster to Earth, located about 54 million light-years away. It contains well over 1,000 galaxies and sits at the center of a larger structure called the Virgo Supercluster, which our own Local Group is part of. The cluster’s gravitational pull is strong enough that it measurably tugs on our galaxy, drawing the Milky Way toward it at several hundred kilometers per second.
The Coma Cluster, roughly five times farther away than Virgo, is the nearest example of a rich, well-organized cluster. It was the first cluster where astronomer Fritz Zwicky noticed that galaxies were moving too fast to be held together by visible matter alone, leading him to propose the existence of dark matter in 1933. Coma contains thousands of galaxies and serves as a benchmark that astronomers use when studying cluster properties.
The Bullet Cluster, formally known as 1E 0657-56, is famous for a different reason. It consists of two clusters that recently collided and passed through each other. During the collision, the hot gas from each cluster slammed together and slowed down, while the dark matter (which doesn’t interact with gas) sailed right through. This separation between the visible gas and the gravitationally detected dark matter provided some of the most direct evidence that dark matter exists as a distinct substance, not just an illusion caused by modified gravity.
How Astronomers Study Clusters
Because so much of a cluster’s mass is invisible, astronomers rely on indirect methods to map what’s actually there. One of the most powerful techniques is gravitational lensing. A cluster’s enormous mass warps the fabric of space around it, bending light from more distant galaxies behind the cluster. This bending stretches background galaxies into arcs and distorted shapes. By analyzing those distortions mathematically, astronomers can reverse-engineer where the mass is concentrated within the cluster, revealing both the visible matter and the dark matter.
X-ray telescopes provide another window into clusters. Since the intracluster medium glows in X-rays, space observatories can map the distribution and temperature of the hot gas. The temperature and brightness of the gas reveal how much total mass the cluster contains, because hotter gas indicates a deeper gravitational well holding everything together.
Optical and infrared telescopes handle the more straightforward task of counting and cataloging the individual galaxies within a cluster. The James Webb Space Telescope has pushed this work to extraordinary distances. In early 2024, Webb confirmed a galaxy at a redshift of 14.32, meaning its light originated less than 300 million years after the Big Bang. While that particular object is an individual galaxy rather than a cluster, Webb’s infrared sensitivity is allowing astronomers to identify proto-clusters (the earliest ancestors of today’s clusters) forming in the young universe.
Why Clusters Matter for Cosmology
Galaxy clusters are more than cosmic curiosities. Their abundance and distribution across the universe serve as a sensitive test of cosmological models. The number of massive clusters that exist at different points in cosmic history depends on how much dark matter and dark energy the universe contains. If dark energy is accelerating the expansion of the universe (which current evidence strongly supports), that expansion works against gravity and limits how many large clusters can form. By counting clusters at various distances and comparing those counts to theoretical predictions, astronomers can constrain the properties of dark energy and test whether general relativity works correctly at the largest scales.
Clusters also act as natural laboratories for studying how galaxies evolve. Galaxies in dense cluster environments behave differently from isolated galaxies. They tend to be redder, older, and less actively forming new stars, likely because interactions with the hot intracluster gas strip away their fuel for star formation. Studying these environmental effects helps explain the diversity of galaxies we see across the universe.