The question of the universe’s biggest entity is not straightforward because “biggest” depends entirely on the metric of measurement. Size can refer to volume, mass, or spatial extent, each leading to a different astronomical record holder. The largest singular objects, such as stars and black holes, are measured by their physical diameter or gravitational mass. As scales increase, structures become less about gravitational binding and more about vast, clustered distributions of matter across immense cosmological distances.
The Scale of Single Massive Objects
When considering a single massive object, the distinction between physical size and gravitational mass becomes immediately relevant. The largest known stars, classified as red hypergiants, are enormous in diameter but are relatively lightweights in mass compared to their dense stellar cousins. An example is Stephenson 2-18, one of the largest stars known, possessing a radius estimated to be over 2,150 times that of the Sun. If placed at the center of our solar system, its outer edge would extend past the orbit of Saturn.
In contrast to these bloated stellar giants, black holes define “biggest” by mass and gravitational influence rather than volume. The ultramassive black hole TON 618 holds the current record for mass, estimated to be 66 billion times the mass of the Sun. This behemoth is the power source of a distant, hyper-luminous quasar, a type of active galactic nucleus. While its event horizon, the point of no return, spans thousands of astronomical units, the black hole itself remains smaller than the largest galaxies that follow.
Defining Cosmological Structures
The next scale involves gravitationally-bound collections of stars and gas that form galaxies. The largest known single galaxy is IC 1101, a supergiant elliptical galaxy found in the Abell 2029 cluster. This galaxy spans an estimated 4 to 6 million light-years in diameter, containing perhaps 100 trillion stars. This structure dwarfs our own Milky Way, which is only 100,000 light-years across.
Galaxies group together into clusters, and these clusters merge into Superclusters. Our local Supercluster is Laniakea, a massive structure stretching approximately 520 million light-years. Laniakea is home to roughly 100,000 galaxies, including our own Local Group. Superclusters represent the largest structures thought to be loosely held together by gravity.
The Known Superstructures and Filaments
The largest contenders for the title of “biggest thing” are the vast, non-gravitationally bound structures that form the large-scale architecture of the cosmos, often referred to as the Cosmic Web. This web consists of dense filaments and walls of galaxies separated by enormous, relatively empty voids. These structures are so immense that they challenge the Cosmological Principle, the foundational idea that the universe is homogenous and isotropic on the largest scales.
The current largest known structure in the observable universe is the Hercules–Corona Borealis Great Wall (HCB GW). This superstructure is a suspected filament of galaxies and gamma-ray bursts that extends for approximately 10 billion light-years in length. For context, the Huge-Large Quasar Group (Huge-LQG) was a former record holder, stretching about 4 billion light-years across.
The HCB GW was discovered by mapping the distribution of gamma-ray bursts, which mark the locations of massive star formation. This 10-billion-light-year length is roughly one-tenth the diameter of the entire observable universe. The existence of such a massive structure poses a theoretical problem, as the universe’s age may not be sufficient for gravity to have formed an object of this size.
The Ultimate Boundary
The ultimate boundary of our measurement is the Observable Universe, which represents the spherical region of space from which light has had time to reach Earth since the Big Bang. This limit is defined by the finite speed of light and the age of the universe. Due to the expansion of space, objects that emitted light 13.8 billion years ago are now much farther away than their light-travel time suggests.
The Observable Universe currently has an estimated diameter of about 93 billion light-years. This figure represents the largest measurable volume in existence, containing all the galaxies, filaments, and voids we can ever hope to see. While the universe may extend far beyond this boundary, the Observable Universe is the definitive scale for the largest thing that is scientifically accessible to us.