The question of the biggest thing in space does not have a simple answer because the concept of “biggest” changes dramatically depending on the scale being measured. At the smallest scale, “biggest” refers to single, physically defined objects, such as stars or black holes. Moving up, the definition shifts to gravitationally bound systems, which are vast collections of these objects like galaxies and superclusters. Finally, the largest known structures are immense, non-gravitationally bound configurations of matter that map the universe’s ultimate architecture.
Defining the Biggest Individual Objects
When considering a single object, the comparison must distinguish between physical diameter and total mass. The largest stars, known as red hypergiants, represent the upper limit of physical size for a single celestial body. These stars swell to tremendous diameters as they near the end of their life cycles, with their outer layers expanding far beyond their dense cores.
The star UY Scuti is often cited as one of the largest stars discovered, possessing a radius estimated to be around 1,700 times that of the Sun. If UY Scuti were placed at the center of our solar system, its volume would engulf the orbits of Mercury, Venus, Earth, and Mars, potentially extending beyond the orbit of Jupiter. However, the exact size of these hypergiants is difficult to measure accurately because their outer layers are diffuse and pulsate over time.
Black holes offer a different measure of “biggest,” defined by the event horizon—the boundary from which nothing can escape. The size of this boundary is directly proportional to the black hole’s mass, meaning the most massive black holes have the largest event horizons. Ultramassive black holes (UMBHs) are a class that exceed 10 billion solar masses.
The black hole at the center of the galaxy cluster Phoenix A is one of the most massive examples, with an estimated mass of 100 billion times that of the Sun. This immense mass creates an event horizon with a diameter of approximately 366 billion miles, nearly 100 times the diameter of Pluto’s orbit. Even though the actual singularity is infinitesimally small, the physical boundary of the black hole’s influence is enormous, representing the largest single physical boundary in the universe.
The Largest Structures Held Together by Gravity
The next step up in scale involves structures held together by mutual gravitational attraction: galaxies, galaxy clusters, and superclusters. Our own Milky Way galaxy, about 100,000 light-years across, is dwarfed by the largest known examples of these systems.
The galaxy IC 1101, a supergiant elliptical galaxy, is one of the largest single galaxies known, with a diameter estimated to be up to 4 million light-years. This structure is nearly 40 times wider than the Milky Way and contains an estimated 100 trillion stars, existing as the brightest member at the core of the Abell 2029 galaxy cluster. Galaxies are grouped together by gravity into clusters and superclusters, forming a cosmic hierarchy.
Galaxy clusters, like the Coma Cluster, contain hundreds to thousands of galaxies, bound tightly enough by gravity to remain a cohesive unit for billions of years. Superclusters represent the largest structures where gravity remains the dominant organizing force. Our own galactic neighborhood resides within the Laniakea Supercluster, which means “immense heaven” in Hawaiian.
The Laniakea Supercluster spans over 520 million light-years and contains approximately 100,000 large galaxies, including the Milky Way. Astronomers define the boundary of Laniakea by mapping the “flow” of galaxies moving toward a common gravitational center known as the Great Attractor. This flow pattern confirms the structure as a single, gravitationally influenced system, though it will eventually be pulled apart by the universe’s expansion.
The Ultimate Scale: Cosmic Walls and Filaments
Beyond the superclusters, the largest structures are not fully bound by gravity but are vast concentrations of matter that trace out the web-like structure of the cosmos. This “cosmic web” consists of enormous voids—regions nearly empty of galaxies—separated by long, thin filaments and walls, which are sheet-like collections of these filaments. These structures are so large that their size is influenced more by the expansion of the universe than by internal gravity.
Early examples of these sheet-like structures include the Sloan Great Wall, discovered using data from the Sloan Digital Sky Survey. This galactic wall measures approximately 1.37 billion light-years in length and contains hundreds of thousands of galaxies and clusters. While its existence initially challenged cosmological models, it was soon surpassed by structures of even greater scale.
The current record-holder for the largest known structure is the Hercules-Corona Borealis Great Wall (HCB GW). This immense filament of galaxies was identified through the mapping of gamma-ray bursts (GRBs), powerful explosions that act as beacons for distant, massive star formation. Initial estimates placed its length at around 10 billion light-years, making it nearly one-tenth the diameter of the observable universe.
The HCB GW’s immense size poses a significant challenge to the Cosmological Principle, which suggests the universe should appear uniformly structured on scales larger than about 1.2 billion light-years. The formation of a structure 10 billion light-years long would have required more time than the age of the universe allows under current standard models. It represents the largest identifiable structure in space, defined as the ultimate, largest-scale organization of matter.