The question of the biggest thing in the universe is complex because “biggest” can refer to sheer volume, total mass, or the maximum distance spanned by a structure. To explore this cosmic hierarchy, it is helpful to start with single, self-contained bodies and move outward toward structures defined by their colossal scale and the gravitational forces that organize them.
The Largest Individual Objects
The largest single objects are defined by either volume or mass, as these two metrics do not always correlate. The champions of volume are red hypergiant stars, which swell to incredible sizes in their late life stages. The star Stephenson 2-18, for example, has a radius estimated to be 2,150 times that of the Sun. If placed at the center of our solar system, its outer atmosphere would extend beyond the orbit of Saturn.
In contrast, the largest objects by mass are supermassive black holes located at the centers of galaxies. The quasar TON 618 is powered by a black hole estimated to be 66 billion times the mass of the Sun. This immense mass creates an event horizon so large that a beam of light would take several weeks to cross it, a scale that dwarfs the entire orbital path of our solar system. The mass contained within these single points of gravity is far greater than the collective mass of all the stars in the Milky Way galaxy.
Groups of Galaxies and Clusters
Moving beyond individual objects, the next tier of cosmic organization involves gravitationally bound collections of galaxies. Our own Milky Way is a member of the Local Group, a modest collection of approximately 85 galaxies spread across a diameter of about 10 million light-years. The two largest members, our galaxy and the Andromeda galaxy, dominate the group’s total mass and are currently on a collision course.
Scaling up significantly, a galaxy cluster is a much denser and more massive assemblage, containing hundreds or thousands of galaxies. The Virgo Cluster, our nearest large cluster, hosts an estimated 1,300 to 2,000 galaxies and spans roughly 15 million light-years across. It exerts a powerful gravitational influence on the entire Local Group, causing our galaxy to accelerate toward it.
Even larger and more populous is the Coma Cluster, which contains over 1,000 major galaxies and extends for about 25 million light-years. Observations of the rapid movement of galaxies within this cluster provided some of the earliest evidence for the existence of unseen dark matter. These clusters are the largest structures in the universe considered to be fully gravitationally bound, meaning their internal gravity is strong enough to resist the overall expansion of space.
The Gigantic Scale of Cosmic Architecture
The next level of organization is the supercluster, which is a collection of galaxy groups and clusters, often connected by immense filaments. Our local supercluster is named Laniakea, a structure containing approximately 100,000 galaxies and stretching over 520 million light-years. The boundaries of Laniakea are defined by the gravitational flow of its member galaxies toward a central region known as the Great Attractor.
Superclusters themselves are woven into the larger structure of the Cosmic Web, a vast, foam-like scaffolding that defines the universe’s large-scale architecture. This web consists of dense filaments and sheets of galaxies, separated by immense, nearly empty regions called cosmic voids. The overall shape of this web is primarily dictated by the gravitational influence of dark matter, which forms the invisible structure upon which visible matter collects.
Within the Cosmic Web are the largest known structures, which are typically long, planar arrangements of galaxies known as galaxy walls or filaments. The Sloan Great Wall, discovered in 2003, is a massive filamentary complex spanning approximately 1.37 billion light-years in length. This structure was considered the largest for nearly a decade, representing a significant challenge to cosmological models due to its immense size.
The current record holder for the largest known structure is the Hercules–Corona Borealis Great Wall, a galaxy filament estimated to be nearly 10 billion light-years long. The size of this structure represents a potential challenge to the Cosmological Principle, which suggests that matter should be uniformly distributed on the largest scales. Its existence, inferred from a clustering of gamma-ray bursts, indicates that the organization of galaxies can extend across a significant fraction of the entire observable universe.
Defining the Ultimate Boundary
The largest concept in the universe is the Observable Universe, which represents a boundary of what we can currently see, rather than a physical edge to space itself. This horizon is defined by the finite speed of light and the age of the universe, which is approximately 13.8 billion years. Light from objects farther away has simply not had enough time to reach us since the Big Bang.
The ongoing expansion of space, however, means that the most distant objects we can observe are now much farther away than the distance light has traveled. Accounting for this expansion, the Observable Universe is estimated to have a diameter of about 93 billion light-years. This boundary is a light-travel limit centered on Earth, meaning any observer in the universe has their own unique observable sphere.
The total universe is thought to be far larger than this observable volume, and may even be infinite, possessing no true physical edge. The observable region is therefore just a small, spherical bubble of space-time accessible to our telescopes. The universe beyond this boundary is unobservable because the light from those regions has been stretched by cosmic expansion and is still traveling toward us.