When considering the largest thing in the universe, the answer is far more immense and complex than a single star or galaxy. The true record holders are not individual objects but colossal structures woven from countless galaxies, stretching across billions of light-years. Answering this question requires shifting focus from local, gravitationally bound bodies to the vast, interconnected scaffolding that makes up the cosmos. Defining “biggest” is difficult, as it can refer to mass, volume, or sheer distance, making the comparison between a star and a galactic wall challenging.
Defining Cosmic Scale
To measure the monumental sizes found in the cosmos, astronomers use specialized units that dwarf terrestrial measurements. The most common is the light-year, which represents the distance light travels in one Earth year, approximately 5.88 trillion miles. Another frequently used unit is the parsec, equal to about 3.26 light-years, with a megaparsec (Mpc) representing one million parsecs.
A direct comparison between objects of different types is impossible. A star’s size is measured by its physical radius or volume, while a galaxy cluster’s size is measured by its spatial extent. This distinction highlights the difference between single, gravitationally-bound entities and the immense, less-dense structures formed by the distribution of matter across space.
Largest Individual Stellar and Galactic Objects
The first impulse when seeking the largest known object is often to look at stars. Among the most massive stars by volume is the red supergiant Stephenson 2-18, whose estimated radius is around 2,150 times that of our Sun. If placed at the center of our solar system, its outer layers would extend past the orbit of Saturn. However, the size of such distant stars is difficult to measure precisely, and some estimates for stars like UY Scuti are now considered less reliable.
Scaling up from stars, the next level of size is the galaxy. The largest known by extent is the supergiant elliptical galaxy IC 1101, located at the center of the Abell 2029 galaxy cluster, about 1.04 billion light-years away. While the Milky Way is about 100,000 light-years across, estimates for IC 1101’s diameter range from 2 million to possibly 6 million light-years. This single galaxy contains an estimated 100 trillion stars.
Structures of Clustered Matter
Beyond individual galaxies, matter organizes itself into hierarchical groupings, beginning with galaxy clusters and then moving to superclusters. A supercluster is a massive collection of galaxy groups and clusters, loosely bound by gravity and moving toward a common center of mass. Our own cosmic neighborhood is part of the Laniakea Supercluster, Hawaiian for “immense heaven,” which contains approximately 100,000 galaxies, including the Milky Way. Laniakea spans about 520 million light-years and is defined by the flow of galaxies within it toward the Great Attractor.
On an even larger scale is the Shapley Supercluster, located about 650 million light-years away in the constellation Centaurus. This structure is one of the most massive and densest concentrations of galaxies in the nearby universe. It stretches over 200 million light-years and is thought to be gravitationally pulling the Laniakea Supercluster toward it. These superclusters are nodes in the vast, sponge-like structure of the universe known as the cosmic web.
The Ultimate Cosmic Record Holders
The ultimate record holders for size are not spheres of matter but enormous, web-like formations of galaxies called filaments or walls. These structures are not gravitationally bound like a star or a galaxy, but represent the largest known overdensities of matter in the cosmos. The largest structure currently known is the Hercules–Corona Borealis Great Wall (HCB Great Wall), a colossal galaxy filament discovered by mapping the distribution of gamma-ray bursts (GRBs).
These GRBs, which are extremely powerful explosions from distant, massive stars, act as cosmic beacons that trace the location of large amounts of matter. The HCB Great Wall is estimated to be approximately 10 billion light-years long, representing nearly one-tenth the diameter of the entire observable universe. Its immense scale directly challenges the cosmological principle—the idea that matter should be evenly distributed on the largest scales. This suggests that either the principle requires revision or our models of the universe’s evolution are incomplete.