Look at a thin slice of brain tissue under a microscope, stained so the neurons stand out. Then look at a picture of the universe on the largest scales, the kind cosmologists make with supercomputers. For a moment it is hard to tell which is which. Both show bright knots joined by thin threads, with dark, empty spaces between them. One picture covers a few millimetres. The other covers hundreds of millions of light-years.
People have noticed this resemblance for years, usually as a pretty coincidence. This piece takes it more seriously. It asks what scientists have actually measured when they compare the two webs, where the likeness holds, and where it falls apart. It also asks the bigger question behind the pictures: what it means that one of these webs, three pounds of living tissue, has managed to map the other.
A word on method before we begin. Comparing the brain and the cosmos invites loose talk, and both neuroscientists and cosmologists are right to be wary of it. Wherever possible I give the actual numbers and name the studies they come from. Where a comparison is only a matter of how things look, I say so. Where it breaks down, I say that too. An honest version of this story turns out to be more remarkable than the loose one.
Part One: Two Webs
The cosmic web
On the largest scales, matter in the universe is not spread out evenly, and it is not scattered at random either. It is arranged in a vast network that cosmologists call the cosmic web. Galaxies gather in dense clusters. Long filaments of galaxies and gas run between the clusters. Thin sheets, or walls, stretch between the filaments. Between all of these lie the voids, huge regions with very few galaxies.
The shape of this web was worked out over decades by mapping where galaxies are. The Sloan Digital Sky Survey was designed to measure spectra, and from them distances, for about a million of the brightest galaxies across roughly a quarter of the sky [1]. Maps like these brought into view structures of startling size. The Sloan Great Wall, a sheet of galaxies found in Sloan data, is about 1.37 billion light-years long [2].
The theory behind the web came into focus in 1996. J. Richard Bond, Lev Kofman and Dmitry Pogosyan showed that the pattern of filaments seen in simulations is already present, in faint form, in the tiny density ripples of the early universe. Gravity does not invent the web. It sharpens a pattern that was there from the start, and the strongest filaments form between nearby clusters that happen to be lined up with each other [3].
How the mass is shared out across the web is lopsided. One detailed analysis of simulations found that voids take up about 77 percent of the volume of the universe but hold only about 15 percent of its mass. Filaments take up only about 6 percent of the volume but hold about half of all the mass [4]. Most of space is close to empty. Most of the matter sits on the threads.
The neural web
The brain is also a web of knots and threads. Its knots are neurons, and its threads are axons and dendrites, the fibres that carry signals out of a cell and into it. Where one neuron meets another there is a synapse, a tiny junction where signals pass across.
Neuroscientists call the full map of these connections the connectome. In one of the papers that introduced the term, in 2005, Olaf Sporns, Giulio Tononi and Rolf Kötter argued that we cannot understand the brain without a structural description of how its parts are wired together [5]. The connectome can be studied at different levels: single synapses, local circuits, or the large tracts of white matter that link distant regions.
Like the cosmic web, the brain's web is neither regular nor random. It has dense local neighbourhoods, a small number of very well-connected hubs, and long-range links that tie distant regions together. That middle ground between order and randomness is where both of our webs live, and it is the first hint that the resemblance might be more than skin deep.
Part Two: Counting the Uncountable
Neurons and galaxies
For years, textbooks said the human brain had 100 billion neurons. In 2009, Frederico Azevedo, Suzana Herculano-Houzel and colleagues actually counted them. They used a method called the isotropic fractionator, which turns brain tissue into a suspension of cell nuclei that can be sampled and counted. The adult male brains they studied held on average about 86 billion neurons and about 85 billion non-neuronal cells [6]. Only about 19 percent of the neurons were in the cerebral cortex. Most of the rest were packed into the cerebellum [6].
The number of galaxies is harder to pin down. In 2016, Christopher Conselice and colleagues used deep surveys and models of the faint galaxies too small to see, and estimated that the observable universe holds about two trillion galaxies [7]. That figure depends on how small a galaxy you are willing to count, and it is still being tested. Later measurements of the faint background glow of the cosmos, made from the New Horizons spacecraft far beyond the glare of the inner solar system, found about half as much light as the two-trillion estimate predicted. NASA has stressed that this result limits the total light from galaxies, not their total number [8].
When Franco Vazza and Alberto Feletti compared the two systems, they used a fairer match. Counting only galaxies at least as massive as our own Milky Way, they put the number at up to about 50 billion. By that measure, both systems have somewhere between ten billion and a hundred billion main nodes [9]. That is a striking coincidence of order of magnitude between a single human head and the observable universe.
Synapses and threads
The brain pulls ahead when we count connections. Bente Pakkenberg and colleagues estimated that the human neocortex alone contains about 0.15 quadrillion synapses, or 150 trillion [10]. If the two-trillion galaxy estimate is right, the synapses in your neocortex outnumber the galaxies in the observable universe by roughly seventy-five to one.
The same group measured the total length of myelinated nerve fibres, the insulated cables of the white matter. In young adults it came to between 150,000 and 180,000 kilometres [10]. A follow-up study found that this length falls by about 45 percent between the ages of 20 and 80, with thinner fibres lost first [11]. At its peak, the cabling packed inside one skull would stretch close to halfway to the Moon.
Each synapse is also a small, adjustable device. Thomas Bartol and colleagues reconstructed synapses in the rat hippocampus in fine detail. They estimated that a synapse can be set to at least 26 distinguishable strengths, which works out to about 4.7 bits of information per synapse [12]. The brain's web is not a static pattern. It is a writable memory.
Part Three: Measuring the Resemblance
Beyond the side-by-side picture
In 2020, an astrophysicist and a neurosurgeon decided to stop comparing pictures and start comparing numbers. Franco Vazza, who studies the cosmic web at the University of Bologna, and Alberto Feletti, a neurosurgeon at the University of Verona, published their study in Frontiers in Physics under the title "The Quantitative Comparison Between the Neuronal Network and the Cosmic Web" [9].
Their materials were deliberately matched. On one side were thin slices of human cerebral and cerebellar cortex, taken during surgery and stained for neurofilaments, the structural proteins inside neurons. On the other side were thin slices cut from a high-resolution simulation of the cosmic web. The slicing of the simulated universe was designed to mimic the way brain tissue is sectioned for the microscope [9].
The power spectrum
The first tool they used comes straight from cosmology: the power spectrum. It breaks a pattern down into waves of different sizes and measures how much of the pattern's variation sits at each size. Cosmologists use it to describe how matter clumps on different scales. It can be used on any image with enough structure.
The result was striking. When brain samples on scales from about 0.01 to 1.6 millimetres were set against the dark-matter distribution of the cosmic web on scales from about 1 to 100 megaparsecs, their spectra agreed closely across nearly two orders of magnitude in scale [9]. To line the two up, the authors had to scale one system to the other by a factor of about 10^27 [9]. Both showed broken power laws, which means their structure changes character at certain scales. That is not what you would expect from a simple fractal [9].
The authors also ran a check. They computed the same spectra for other natural patterns: clouds, tree branches, water turbulence and turbulence in magnetised plasma. Those systems showed simpler, more regular spectra, unlike the brain and the cosmic web [9]. The resemblance was not something the method produced for any branching pattern you gave it.
Nodes, links and clusters
Two quite different patterns can share the same spectrum, a limit the authors pointed out themselves [9]. So they also built networks. They picked out the densest points in each image as nodes, linked nodes that lay within a set distance of one another, and measured how the resulting networks were organised.
The cosmic-web slices yielded about 3,800 to 4,700 nodes, each with on average about four connections. Cortex samples gave about 4.6 to 5.4 connections per node, and cerebellum samples about 1.9 to 3.7 [9]. Both kinds of network were highly clustered, with clustering coefficients peaking between about 0.1 and 0.4. In a random network of the same size, the clustering coefficient would be about 0.002 or less [9]. In both cases, neighbours of a node tend to be linked to each other. Both are far from random.
Vazza and Feletti were careful about what this means. Their conclusion was that similar network shapes can arise from entirely different physical processes, which hints at shared principles of self-organisation "despite the radically different scales and processes at play" [9].
Two curious coincidences
The paper notes two further parallels, and offers both with caution. The first is about composition. About 77 to 78 percent of the brain is water. In the cosmological model the authors used, about 73 percent of the universe's energy is dark energy [9]. In both cases, roughly three quarters of the system is a pervasive medium that shapes the internal structure only indirectly. More recent measurements from the Planck satellite put dark energy at about 68.5 percent [13], so the match is loose. It is a poetic rhyme, not a law.
The second is about memory. Using Bartol's estimate of synaptic information, the authors put the brain's total storage capacity at about 2.5 petabytes. A separate information-theory estimate suggested that about 4.3 petabytes would be needed to store the structure of the cosmic web across the entire observable universe [9]. The authors themselves say this "may appear as a mere coincidence," given how roughly both numbers are known [9]. It is worth pausing on anyway. By this one crude measure, the information in the structure of a human brain and in the structure of the visible cosmos are of the same order.
Part Four: Hubs, Clubs and Small Worlds
The small world
In 1998, Duncan Watts and Steven Strogatz described a kind of network that sits between perfect order and pure randomness. In their models, a regular lattice gained a few random shortcuts. The result was a network that stayed highly clustered, like the lattice, yet had short paths between any two points, like a random graph. They called these "small-world" networks. One of their first real examples was the nervous system of the tiny worm Caenorhabditis elegans [14].
Over the next two decades, a large body of work, reviewed by Danielle Bassett and Ed Bullmore, showed that human brain networks have this small-world character too [15, 16]. Ed Bullmore and Olaf Sporns built the wider framework for analysing the brain as a graph, using the tools of network science to study both its structure and its activity [17]. The appeal of the small world is clear. Local clustering lets specialised regions do focused work. Short paths let the whole brain pull that work together quickly.
The rich club
In 2011, Martijn van den Heuvel and Olaf Sporns found something more specific. Mapping the white-matter networks of 21 people with diffusion imaging, they identified 12 hub regions spread across both hemispheres, including the precuneus, the superior frontal and superior parietal cortex, the hippocampus, the putamen and the thalamus. These hubs were more densely linked to each other than their number of connections alone would predict. They formed what network scientists call a "rich club" [18].
A year later, the same group showed what the rich club does. Its connections were costly, accounting for about 40 percent of the brain's total wiring cost. But about 69 percent of the shortest paths between pairs of brain regions passed through it [19]. The rich club is the brain's central backbone, a small elite of hubs that carries much of its long-distance traffic.
Hubs in the cosmos
The cosmic web also has hubs. Massive galaxy clusters sit at the junctions of several filaments, and matter streams along those filaments toward them. Our own galaxy belongs to a larger structure that astronomers have mapped by tracing how galaxies move. Brent Tully and colleagues found the boundaries where these flows divide, "as water does at watershed divides," and named the region inside them Laniakea, our home supercluster [20]. The universe, like the brain, is organised into basins that drain toward dense centres.
In the brain, a hub is a hub because of what it does: it routes signals and integrates information. In the cosmos, a cluster is a hub only in the sense that matter collects there. The shapes rhyme. The jobs do not. That difference is the heart of the next part of this story.
Part Five: The Invisible Majority
Dark matter as scaffolding
Most of the matter in the cosmic web cannot be seen. According to the Planck satellite's measurements, ordinary matter makes up only about 5 percent of the universe's energy content, while dark matter makes up about 27 percent [13]. Dark matter does not shine or absorb light. It reveals itself through gravity.
In the standard picture, dark matter forms the scaffolding of the web. The Millennium Simulation, run by Volker Springel and the Virgo Consortium and published in 2005, followed about ten billion particles of dark matter in a cube about 2.2 billion light-years on a side, from shortly after the Big Bang to the present [21]. The scaffolding that formed in that simulated universe gave galaxies a place to form. Ordinary gas fell into the dark matter's wells, cooled, and lit up as stars [21]. The visible galaxies are like lights strung along an unseen frame.
The brain's quiet partners
The brain also has a large population of cells that do not fire the way neurons do. Azevedo and colleagues counted about as many non-neuronal cells as neurons [6]. These glial cells insulate axons with myelin, regulate the chemical environment, clear away waste and support the work of synapses.
It is tempting to call glia the brain's dark matter. The comparison works only up to a point. Glia, like dark matter, are a large and once-overlooked population that shapes the structure everyone else sees. But glia are anything but invisible. They are living cells that can be stained, imaged and studied directly, and they take part in brain chemistry in ways dark matter, as far as anyone knows, never could. The metaphor is useful as an invitation to look closer. It should not be taken as a finding.
Part Six: Where the Analogy Breaks
Scale and time
The two webs differ in size by about 27 orders of magnitude [9]. They also differ wildly in time. A neuron can fire, and a thought can form, in a fraction of a second. The cosmic web has been taking shape over the universe's whole history, about 13.8 billion years [13]. Light itself needs tens of millions of years or more to travel the length of a large filament. In the time it takes you to read this sentence, your brain has exchanged countless signals. In the same span, the cosmic web has not perceptibly changed.
Dynamics and causation
The forces that build each web have nothing in common. The cosmic web is shaped mainly by gravity acting on dark and ordinary matter in an expanding universe. Its pattern was largely written into the early universe and has been sharpened ever since [3]. The brain's web is built by genes, cell chemistry and electrical activity. It is shaped by evolution over millions of years, by development in the womb and in childhood, and by experience every day of a life. It keeps rewriting itself. When you learn a name, some of your synapses change. Nothing like that happens in a filament of galaxies.
Signals and meaning
This is the deepest difference. Neurons signal to one another. They send electrical and chemical messages that carry information, and the network's structure exists to route and transform those messages. The threads of the cosmic web do not carry messages. Filaments feed gas into galaxies and guide the flow of matter, but no galaxy is computing anything about another. A connection in the brain is a channel. A connection in the cosmic web is a stream of matter falling under gravity.
The limits of the method
Vazza and Feletti were open about the limits of their study. They looked at small two-dimensional slices, not whole three-dimensional systems. They defined nodes by where stain was densest, which is only an approximation of where cell bodies are. Above all, they linked nodes by proximity alone. That suits the cosmic web, but it misses the long-range connections that are among the most important features of the brain [9]. A resemblance in local structure is real, but it is not a resemblance in function.
None of this cancels the finding. Two systems built by completely different forces are measurably similar in some structural statistics. That points toward something general about how complex networks organise, not toward a hidden link between your neurons and the galaxies.
Part Seven: Twenty Watts
A thrifty organ
Here the brain leaves the cosmos far behind. The adult human brain makes up about 2 percent of body weight but uses about 20 percent of the body's oxygen and calories [22]. In absolute terms, that comes to roughly 20 watts, less than the laptop many of us use to read articles like this one [23].
William Levy and Victoria Calvert took a closer look at where that energy goes. They estimated that cortical grey matter uses only about 3 watts for its actual computation and communication [23, 24]. And within that budget, communication, sending signals along axons, costs about 35 times more energy than computation [24]. Most of what the brain spends, it spends on talking to itself.
An economy of wiring
This is where the network structure and the energy budget come together. Ed Bullmore and Olaf Sporns have argued that the brain is shaped by a trade-off between keeping wiring cheap and keeping its network able to do complex work [25]. Short, local connections are cheap. Long-range connections are expensive, but they make fast integration possible. The rich club is the result of that trade-off: a costly backbone that pays for itself by carrying most of the brain's long-distance traffic [19].
The cosmic web is not under any such pressure. It is not trying to do anything, and nothing selects it for efficiency. The brain, by contrast, has been tuned over hundreds of millions of years of evolution to do an extraordinary amount on a budget. When we find small-world structure and a rich club in the brain, we are seeing the signature of that tuning. Whatever language one prefers, design or optimisation or selection, the result is one of the most efficient information-processing systems known.
Part Eight: The Model in the Mind
A brain that predicts
The brain was long described as a machine that builds a picture of the world from incoming sensations. A newer view turns this around. In 1999, Rajesh Rao and Dana Ballard proposed that the visual cortex works by prediction. Higher areas send down predictions of what the lower areas should be seeing. Lower areas send back only the errors, the part of the input the prediction got wrong [26].
Karl Friston expanded this idea into a broad theory. He proposed that perception, action and learning can all be understood as the brain working to reduce its prediction errors, or more formally a quantity he calls free energy [27]. The theory is still debated, but on this view the brain is always running a model of the world and updating it against what the senses report.
A universe in a box
Cosmology works in much the same way. The Millennium Simulation was a model of the universe built from physical theory and a set of starting conditions. It ran on 512 processors for about 28 days, using nearly a terabyte of memory, and its output was then compared with real surveys of galaxies [21]. Where the model and the sky disagree, cosmologists revise the model. It is prediction and error, at the scale of a scientific field.
And every step of it was done by brains. The theory behind the simulation, the code that ran it, the telescopes that tested it and the judgement about what it meant all came from human minds running on about 20 watts each. A twenty-watt network of nerve cells has built a working model of a web of galaxies spanning billions of light-years. Whatever else is true of the comparison between the two webs, this is the fact that should take our breath away.
Part Nine: Star Stuff
Elements forged in stars
The connection between the brain and the cosmos is not only a matter of pattern. It is also a matter of material. In 1957, Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle published a long paper explaining how the chemical elements are built inside stars [28]. Known as B2FH after its authors' initials, it laid out the nuclear processes by which stars fuse light elements into heavier ones and scatter them into space when they shed their outer layers or explode.
Modern work has filled in the picture. The Big Bang made hydrogen, helium and a little lithium. Nearly everything else on the periodic table was made later, by stars and by violent events such as exploding stars and colliding neutron stars [29]. The carbon in every cell membrane, the oxygen in the water that makes up most of the brain, the phosphorus in DNA, the calcium that triggers the release of neurotransmitters at every synapse: nearly all of it was forged in stars that lived and died before the Sun existed.
A way for the cosmos to know itself
Carl Sagan put this into words in the first episode of Cosmos, broadcast in 1980: "We're made of star-stuff. We are a way for the cosmos to know itself" [30]. The first sentence is now a matter of nuclear physics. The second is a matter of perspective. But it is hard to find a better summary of the facts.
The matter of the universe gathered along the filaments of the cosmic web. In the dense knots of that web, galaxies formed. Inside galaxies, generations of stars built the heavier elements and released them. From those elements, on at least one small planet, came living things, and eventually a three-pound organ that could look up, wonder what it was seeing, and work out the answer. The web made the stars. The stars made the elements. The elements made the brain. And the brain mapped the web.
Part Ten: Light, and the Limits of What We Know
The light that reaches the mind
This site is about light, so it is worth ending there. Every map of the cosmic web begins with light: photons that left distant galaxies long ago and travelled across the universe before landing in a telescope. In the end, every one of those images is seen by a human eye. The retina grows out of the developing brain. When starlight reaches it, the cosmos is, quite literally, touching the nervous system.
The same is true each morning. The sunlight that meets the eye near the horizon was made by nuclear fusion in the core of a star, the same family of processes that made the elements of the brain receiving it. When we watch the low sun, starlight is meeting a mind built from the elements that earlier stars made.
What the networks cannot explain
There is one thing no comparison of networks can settle. In 1995, the philosopher David Chalmers separated the "easy" problems of the mind, such as how the brain processes information, from the "hard problem": why any of that processing is accompanied by experience at all [31]. A power spectrum can tell us that the brain and the cosmic web have similar structure. A clustering coefficient can tell us that both are far from random. Neither can tell us why one of them feels like something from the inside, and the other, as far as we know, does not.
That gap is not a failure of the comparison. It is the most important thing the comparison reveals. Two webs, built by different forces across a gulf of 27 orders of magnitude in size, share a surprising amount of their architecture. Only one of them is aware. Only one of them asks questions. And only one of them, on a budget of about 20 watts, has learned to read the other.
Closing Thought
The resemblance between the brain and the cosmic web is real, and it can be measured. It is also limited, and it says more about how complex networks form than about any hidden link between neurons and galaxies. Held honestly, though, the comparison points to something deeper than a pretty picture. The universe built itself into a web, and out of that web came a second web, small enough to fit inside a skull and able to hold a model of the first. We are not outside the cosmos, looking in. We are one of the ways it has found to look at itself.
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