The universe spans roughly 60 orders of magnitude in size, from the smallest meaningful length in physics (the Planck length, at about 10⁻³⁵ meters) to the observable universe (at about 10²⁶ meters). Between those extremes, matter organizes itself into a surprisingly clean hierarchy: subatomic particles nest inside atoms, atoms build molecules, molecules assemble into cells and everyday objects, objects populate planets, planets orbit stars, stars gather into galaxies, and galaxies weave together into a web-like structure that stretches across billions of light-years. Each jump in scale brings different physics into play, and the transitions between levels are sometimes sharp and sometimes blurry.
The Planck Length and Why There Is a Floor
The sequence starts at the bottom with the Planck length, roughly 1.6 × 10⁻³⁵ meters. This is not just a very small number someone picked for convenience. It represents a physical boundary: the energy you would need to probe a region of space smaller than the Planck length would be so enormous that it would collapse into a black hole, making the measurement self-defeating.1Physics Education. Space-time is doomed! Introducing Planck scale physics in the classroom Below this threshold, our current understanding of space and time breaks down entirely. Nothing we know of is “smaller” than the Planck length in any experimentally meaningful sense.
This does not mean the Planck length is a physical object or a building block. It is a limit on resolution, like a pixel size for reality itself. Whatever happens at that scale, and physicists suspect something fascinating does, we do not yet have the theoretical tools to describe it. So the ladder of cosmic structure begins just above that floor.
Quarks, Protons, and Atoms
Step up from the Planck length by about 20 orders of magnitude and you reach the domain of subatomic particles. Quarks, the constituents of protons and neutrons, are smaller than about 10⁻¹⁸ meters based on scattering experiments. Whether quarks have any internal structure at all remains an open question. Electrons appear to be point-like at every scale we have tested.
Protons and neutrons, built from three quarks bound together by the strong force, come in at around 10⁻¹⁵ meters, roughly a femtometer. Atomic nuclei, clusters of protons and neutrons, range from about one to a few femtometers depending on the element. The full atom, including its electron cloud, is dramatically larger: a hydrogen atom measures about 10⁻¹⁰ meters (one angstrom), while heavier atoms like cesium stretch to a few angstroms. Most of the atom is empty space, which is why the nucleus-to-atom jump represents a factor of roughly 100,000 in size.
Molecules to Living Cells
Atoms bond into molecules, and the size range here is staggering. A water molecule spans about 2.75 angstroms. A strand of DNA in a human cell, if uncoiled, would stretch about two meters, but it is packed into a nucleus only about six micrometers across. Proteins fold into shapes measured in nanometers (10⁻⁹ meters). Viruses, which sit at the fuzzy boundary between chemistry and biology, range from about 20 to 300 nanometers.
Living cells push into the micrometer range. A red blood cell is about 7 micrometers in diameter, while some nerve cells can extend axons a full meter in length (though they remain microscopically thin). Cells represent the first level of the hierarchy where the organizing principle shifts from fundamental forces to the messy logic of evolution, and size becomes highly variable depending on the organism and function involved.
Human-Scale Objects and the Logarithmic Pattern
From cells, you enter the world of everyday experience: insects, people, buildings, mountains. A human body is on the order of one to two meters. Mount Everest rises about 8,800 meters above sea level. These seem like mundane reference points, but they sit at a meaningful midpoint in the cosmic scale, roughly halfway (on a logarithmic axis) between the Planck length and the observable universe.
Research on how people perceive spatial scales shows that we organize familiar environments, from rooms to buildings to cities to countries, along transitions that follow a logarithmic increase in size. When study participants mapped out familiar locations at six different scales, from rooms to continents, the distances between items at each level fit a logarithmic curve with remarkable consistency.2eLife. Processing of different spatial scales in the human brain Our brains handle the enormous range of scales we encounter daily by compressing them logarithmically, the same way the cosmic hierarchy itself is best understood on a logarithmic ruler rather than a linear one.
Planets and the Solar System
Moving up from human-scale objects, we reach planets. Earth measures about 12,742 kilometers in diameter. Jupiter, the largest planet in our solar system, is roughly eleven times wider. The Sun dwarfs them all at about 1.4 million kilometers across, large enough that you could line up about 109 Earths across its face.
The solar system itself is far larger than the Sun, but defining its “edge” is surprisingly tricky. If you count the orbits of the planets, Neptune’s orbit extends to about 30 astronomical units from the Sun (one AU is the Earth-Sun distance, about 150 million kilometers). But the solar system’s gravitational influence reaches much farther. The Oort cloud, a vast shell of icy bodies surrounding the solar system, extends to distances of up to 200,000 AU from the Sun.3Universe. Oort Cloud Comets: Perturbations Due to the Passage of Gliese 710 That is roughly 3.2 light-years, a sizable fraction of the distance to the nearest star.
The Oort cloud has never been directly observed. It is inferred from the orbits of long-period comets that swing into the inner solar system from every direction, suggesting a roughly spherical source reservoir. Detecting individual Oort cloud objects may eventually be possible through stellar occultation surveys, where an object briefly blocks a distant star’s light.4The Astronomical Journal. An Efficient Observational Strategy for the Detection of the Oort Cloud For now, the Oort cloud remains a theoretically solid but observationally unconfirmed part of the hierarchy, a reminder that even within our own solar system, vast regions are still essentially invisible to us.
Stars and Star Clusters
Beyond individual solar systems, the next rung up is groups of stars. Stars do not form in isolation. They are born in clusters from the same collapsing cloud of gas and dust, and these clusters can hold anywhere from a few dozen to millions of stars.
Open clusters are the smaller variety, loosely bound groups of stars that typically contain a few hundred members. These clusters span a few to a few dozen light-years across and gradually disperse over hundreds of millions of years as their stars drift apart. As an example, one recently studied open cluster was found to contain about 128 member stars with a total mass of a few hundred solar masses and an age of roughly 115 million years.5PubMed Central. Machine learning-enabled membership determination of open cluster AH03_J0748-26.9 using Gaia DR3 astrometry Globular clusters, the denser variety, pack hundreds of thousands to millions of stars into a roughly spherical ball just 50 to 200 light-years across. Many globular clusters are among the oldest objects in the universe, with ages approaching 13 billion years.
Galaxies
Stars, gas, dust, and dark matter collect into galaxies, which represent one of the most visually striking levels of the hierarchy. The Milky Way, our home galaxy, is a barred spiral roughly 100,000 light-years in diameter containing an estimated 100 billion to 400 billion stars. That sounds enormous, but the Milky Way is a middle-of-the-road galaxy. Dwarf galaxies can be as small as a few thousand light-years across and hold only a few billion stars. Giant elliptical galaxies at the other extreme can stretch over a million light-years and contain trillions of stars.
One of the less intuitive facts about galaxies is that they are far closer together, relative to their size, than stars are. The Andromeda Galaxy is about 2.5 million light-years from the Milky Way, which is only about 25 times the Milky Way’s own diameter. By contrast, the nearest star to the Sun is about 4.2 light-years away, more than 30 million times the Sun’s diameter. Galaxies practically crowd each other in cosmic terms, and collisions between them are common over cosmological timescales.
Galaxy Groups, Clusters, and Superclusters
Galaxies are not sprinkled randomly through space. They clump together under gravity into progressively larger associations. The smallest groupings are galaxy groups, typically containing a few to a few dozen galaxies held together gravitationally. The Milky Way belongs to the Local Group, a collection of about 80 known galaxies spread across roughly 10 million light-years.
Galaxy clusters are the next step up, containing hundreds to thousands of galaxies within a region spanning 10 to 30 million light-years. The Virgo Cluster, the nearest large cluster to us, holds over a thousand galaxies. Galaxy clusters are among the largest gravitationally bound structures in the universe, meaning their member galaxies are genuinely orbiting a common center of mass rather than just drifting in the same general direction.
Superclusters are even larger, but here the concept of “gravitationally bound” starts to break down. Superclusters are vast assemblages of galaxy clusters and groups that stretch across hundreds of millions of light-years. The Laniakea Supercluster, which includes the Local Group, spans roughly 500 million light-years. But most of Laniakea is not gravitationally bound; the expansion of the universe is pulling much of it apart faster than gravity can hold it together. Superclusters are therefore less like permanent structures and more like a snapshot of how matter happens to be arranged right now.
The Cosmic Web
Zoom out far enough and the distribution of matter in the universe stops looking random and starts looking architectural. Galaxies, galaxy clusters, and superclusters are arranged in a vast network of dense nodes connected by filamentary structures, with enormous nearly empty voids between them. This pattern is called the cosmic web, and it reflects the fundamental large-scale distribution of matter shaped by gravity over billions of years.6Academic Journal of Science and Technology. The Cosmic Web: Unveiling the Hidden Structure of the Universe through Observational Cosmology
Filaments are the cosmic web’s connective tissue. They are elongated bridges of gas, dark matter, and galaxies that link dense cluster nodes together. One directly observed filament, detected through its hydrogen emission near a quasar at a distance corresponding to a lookback time of about 10 billion years, measured roughly 460 kiloparsecs (about 1.5 million light-years) in projected length and extended well beyond any single galaxy’s dark-matter halo.7PubMed. A cosmic web filament revealed in Lyman-α emission around a luminous high-redshift quasar Filaments can be far longer than this: some stretch for hundreds of millions of light-years. The voids between filaments, where galaxies are extremely sparse, can span 100 to 300 million light-years across.
The cosmic web is not just a curiosity. Most of the ordinary matter in the universe (protons, neutrons, electrons) is thought to reside not inside galaxies but in the diffuse, hot gas that fills filaments, a reservoir sometimes called the warm-hot intergalactic medium. Finding and mapping this gas has been one of the persistent challenges of observational cosmology.
The Observable Universe and What Lies Beyond
The observable universe is the largest structure we can reference with any precision. It is defined not by a physical wall but by a horizon: the maximum distance from which light has had time to reach us since the Big Bang about 13.8 billion years ago. Because the universe has been expanding throughout that time, the actual radius of the observable universe is about 46.5 billion light-years, far larger than the 13.8 billion light-years you might naively expect from the age alone.
This means the diameter of the observable universe is roughly 93 billion light-years. It contains an estimated two trillion galaxies, though that number depends on counting methods and what you consider a galaxy versus a fragment. Beyond the edge of the observable universe, space almost certainly continues, possibly infinitely, but we have no way to observe or measure it. The observable universe is not the “whole” universe; it is just the part accessible to our instruments.
Where the Ladder Gets Wobbly
This hierarchy, Planck length to observable universe, is tidy, but nature does not always respect tidy categories. Several places in the ladder are blurrier than they appear.
At the smallest scales, the boundary between “particle” and “field” dissolves. In quantum field theory, particles are excitations of underlying fields, and asking “how big is an electron” does not have a clean answer. At the largest scales, the concept of distance itself becomes complicated. Because the universe is expanding, the distance between two galaxies changes while the light traveling between them is in flight. Cosmologists use multiple definitions of distance (light-travel distance, comoving distance, angular diameter distance) depending on what question they are asking. Work on the cosmic distance ladder has shown that local inhomogeneities in the distribution of matter can modify measurements of cosmic distances by a non-trivial amount, roughly 11% in some calculations, due to tidal deformation of the local spacetime around an observer.8Journal of Cosmology and Astroparticle Physics. The art of building a smooth cosmic distance ladder in a perturbed universe The universe is not a smooth, uniform place, and our position within a lumpy region of it subtly warps how we measure everything beyond it.
There is also the question of dark matter and dark energy, which together make up about 95% of the universe’s total energy content. Dark matter contributes gravitationally to structures at every level from galaxies upward. Dark energy drives the accelerating expansion of the universe and operates at the very largest scales. Neither fits neatly into a “smallest to largest” hierarchy because they are not objects with a size. They are pervasive, and they shape every rung of the ladder without occupying any single one.
The Hierarchy in Numbers
For a quick reference, here is the approximate size of each major level, expressed in meters on a logarithmic scale:
- Planck length: 10⁻³⁵ m
- Quarks: smaller than 10⁻¹⁸ m
- Protons and neutrons: 10⁻¹⁵ m
- Atoms: 10⁻¹⁰ m
- Viruses: 10⁻⁸ to 10⁻⁷ m
- Human cells: 10⁻⁵ m
- Humans: 10⁰ m (about 1-2 meters)
- Earth: 10⁷ m
- Sun: 10⁹ m
- Solar system (to Oort cloud): 10¹⁶ m
- Milky Way: 10²¹ m
- Galaxy clusters: 10²³ m
- Cosmic web filaments: 10²⁴ to 10²⁵ m
- Observable universe: 10²⁶ m
Each step up covers several orders of magnitude. The gaps are not evenly spaced, and some levels (like the jump from atoms to cells) span more orders of magnitude than others (like the jump from galaxy clusters to the cosmic web). The universe does not build itself in regular increments; it builds in whatever increments the underlying physics allows.
Why “Largest” Might Not Have an Answer
The observable universe is the biggest thing we can put a number on, but it is almost certainly not the biggest thing that exists. Inflation, the rapid expansion thought to have occurred in the universe’s first fraction of a second, may have stretched space far beyond what we can see. Some cosmological models suggest the full universe could be trillions of times larger than the observable portion, or infinite. A few speculative frameworks propose a multiverse, an ensemble of separate universes with potentially different physical laws, but these ideas remain untestable and therefore outside the scope of observational science.
Even within the observable universe, the question “what is the largest structure?” keeps getting revised upward. In the 1980s, the discovery of galaxy superclusters pushed the scale of known structures to hundreds of millions of light-years. In the 2000s and 2010s, surveys identified structures like the Sloan Great Wall (about 1.4 billion light-years long) and the Hercules-Corona Borealis Great Wall (estimated at about 10 billion light-years across), though the statistical significance and physical coherence of these features remain debated. The cosmic web itself, as a single interconnected network, may be the largest coherent pattern in the observable universe, but calling it “a structure” is a stretch when it is really the backdrop against which all other structures sit.