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| Mapping the void: Dark matter and dark energy hold the secrets to 95% of our existence. |
Look at the night sky long enough and a troubling thought takes hold: everything you can see — every star, every smear of galaxy light, every luminous thing in the entire observable universe — adds up to roughly
5 percent of what actually exists. The other 95 percent is invisible. Not invisible in the way a distant planet is invisible, where "invisible" just means "too faint" or "too far." Invisible in a deeper, more unsettling sense: it does not emit light, does not reflect light, does not interact with light at all. For decades, physicists have built instruments of extraordinary sensitivity to catch it, and come up empty. And yet its gravitational fingerprints are everywhere — in the spin of galaxies, in the bending of spacetime, in the large-scale architecture of the entire cosmos. We know it exists. We cannot find it. And as of 2025, there are signs that the second component — the 68 percent called dark energy — may be changing over time in a way that Einstein's equations strictly forbid. If those signs solidify, it will be the most significant shift in cosmology since 1998. The universe, it turns out, is stranger than the 5 percent we can see has ever prepared us to expect.
Limits of the Universe · Episode 02 of 05
The Invisible Universe
Everything humanity has ever seen, mapped, and measured accounts for just 5% of the cosmos. The rest — dark matter and dark energy — is pulling the universe toward a fate we are only beginning to understand.
5% visible · 27% dark matter · 68% dark energy
The Complete Series
EP 02
The Invisible Universe — Dark Matter & Dark Energy
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The Astronomer Who Cried Invisible: Fritz Zwicky and the First Clue
In 1933, Fritz Zwicky was studying the Coma Cluster — a collection of about 1,000 galaxies roughly 320 million light-years from Earth — and something in his numbers refused to work. He measured how fast the individual galaxies within the cluster were moving relative to each other. Then he calculated how much gravitational pull would be needed to prevent them from flying apart. The two figures disagreed by an embarrassing margin. The visible mass of the cluster — all the stars, all the gas, everything Zwicky could see — produced only about one-fiftieth of the gravity required to hold the thing together.
Zwicky proposed the existence of dunkle Materie — dark matter — a form of mass that did not emit light but exerted gravitational pull. His colleagues mostly ignored him. Zwicky had a reputation for being difficult, and his claim was sufficiently extraordinary that the burden of proof was high. For the next four decades, dark matter remained a fringe suggestion carried by one irritable Swiss astronomer and a body of data that most people preferred not to look at too closely.
Vera Rubin's Galaxies and the Rotation Problem That Changed Everything
In the 1970s, Vera Rubin and Kent Ford at the Carnegie Institution began systematically measuring the rotation curves of spiral galaxies — plotting how fast stars at different distances from a galaxy's centre are orbiting. The expectation, based on straightforward Newtonian gravity, was clear: stars far from the centre, where most of the visible mass is concentrated, should orbit more slowly, the way the outer planets of the solar system orbit the Sun more slowly than the inner ones.
Instead, Rubin found that the rotation curves were flat. Stars at the outer edges of galaxies were orbiting just as fast as stars near the centre — sometimes faster. Galaxy after galaxy showed the same pattern. The only explanation that held up under scrutiny was that each galaxy was embedded in a vast, invisible halo of matter extending well beyond its visible boundary, providing the extra gravitational pull that kept the outer stars from flying off into space. The mass contained in this halo was, by Rubin's estimates, at least five to ten times the mass of all the visible stars.
This could not be dismissed the way Zwicky had been. Rubin had data from dozens of galaxies, gathered over years, checked and rechecked. The flat rotation curves were not a measurement error. They were a signal. The universe contained far more matter than anyone had been able to see. Rubin spent the rest of her career accumulating evidence of what she called "the dark side of the universe" — and died in 2016 without receiving the Nobel Prize that much of the physics community believed she had more than earned.
The Bullet Cluster: The Clearest Evidence We Have
In 2006, astronomers published observations of two galaxy clusters that had collided roughly 150 million years ago — a system called 1E 0657-558, quickly nicknamed the Bullet Cluster. What made it remarkable was that the collision had separated its components in a way that gave physics a clean test.
When the two clusters passed through each other, the ordinary matter — the hot gas that constitutes most of the visible mass in galaxy clusters — slowed down because gas interacts electromagnetically: it collides, heats up, and drags. X-ray observations showed this gas piled up between the two clusters, glowing hot from the collision. But the gravitational lensing maps told a different story. The gravitational mass — the mass actually bending the light of background galaxies — had continued right through the collision without slowing down. It was concentrated around the two original cluster cores, not in the hot gas between them.
The implication was unambiguous. The dominant mass component of these clusters did not interact electromagnetically. It passed through the collision as though the other cluster were not there. This is precisely what dark matter would do — and it is extremely difficult to explain any other way. The Bullet Cluster is widely considered the most direct observational evidence that dark matter exists as a physical component of the universe, separate from ordinary matter, and not merely a sign that our understanding of gravity is incomplete.
What the Bullet Cluster shows: Ordinary matter (gas) slowed down and piled up during the collision. Dark matter passed straight through. Gravitational lensing maps confirm the mass distribution followed the dark matter, not the gas. Two things in the same space, interacting with gravity but not with each other or with light.
What Dark Matter Might Actually Be
For most of the last thirty years, the leading candidate was the WIMP — the Weakly Interacting Massive Particle. WIMPs are theoretically attractive for a compelling reason: the same extensions of the Standard Model of particle physics that were proposed to solve completely unrelated problems in quantum mechanics naturally predict particles with exactly the mass and interaction strength that dark matter requires. This coincidence — physicists call it the "WIMP miracle" — made WIMPs feel almost inevitable. The universe seemed to be winking at us.
The WIMP miracle has not survived contact with experiment. Over three decades, increasingly sensitive detectors have failed to find a single confirmed WIMP event. The parameter space — the range of masses and interaction strengths within which WIMPs could hide — has been systematically eliminated to the point where the original, most natural WIMP models are essentially ruled out. The universe's wink turned out to be a coincidence, or at least an incomplete one.
Other candidates remain in serious contention. Axions are extraordinarily light particles originally proposed in the 1970s to solve a different problem in quantum chromodynamics. They interact so weakly that they could pass through a light-year of lead without being absorbed. Experiments like ADMX are searching for them using resonant cavities tuned to the frequency at which axions would convert to microwave photons. Sterile neutrinos are hypothetical heavy relatives of the known neutrinos, which could be produced in the early universe in the right quantities to account for dark matter. Primordial black holes — black holes formed in the first fraction of a second after the Big Bang — are being constrained by gravitational microlensing surveys but have not been ruled out entirely in certain mass ranges.
Major Dark Matter Candidates
| Candidate |
Key Property |
Status (2025) |
| WIMP |
Weak force interaction, ~GeV–TeV mass |
Most natural range largely ruled out |
| Axion |
Ultra-low mass, electromagnetic conversion |
Actively searched; not detected |
| Sterile Neutrino |
Heavy neutrino, no Standard Model force |
Constrained but viable |
| Primordial Black Hole |
Macroscopic, gravitational only |
Some mass ranges still open |
The Hunt: LZ, XENONnT, and Three Decades of Silence
The most sensitive dark matter search ever conducted operates nearly a mile underground at the Sanford Underground Research Facility in Lead, South Dakota. The LUX-ZEPLIN experiment — LZ — fills a titanium vessel with ten tonnes of liquid xenon, cooled to −96°C, surrounded by layers of shielding designed to block out every conceivable source of background radiation. It sits underground because cosmic rays are one of the most persistent sources of false signals, and rock is still the most effective shield.
In December 2025, LZ published results from 417 live days of data — the largest dataset ever collected by a dark matter detector. The analysis found no sign of WIMPs with masses between 3 and 9 GeV/c² — the first time LZ had probed this low-mass range, and a world-leading sensitivity above 5 GeV/c². The experiment is, according to its spokesperson Rick Gaitskell of Brown University, more than three million times more sensitive than the instruments used when he began working in this field. It found nothing. What it did detect, for the first time, were boron-8 solar neutrinos interacting with xenon nuclei through a rare quantum process — a milestone in sensitivity that proved the detector was working exactly as designed. Dark matter remained silent.
XENONnT at the Gran Sasso laboratory in Italy, PandaX-4T in China's Jinping Underground Laboratory — the three largest liquid xenon experiments in the world are now closing in on the so-called "neutrino floor," the sensitivity threshold below which neutrino interactions become an irreducible background that mimics dark matter signals. Reaching the neutrino floor without a detection would not definitively rule out WIMPs, but it would mean that any remaining WIMPs interact so weakly that distinguishing them from solar neutrinos becomes extraordinarily difficult. Future detectors on the scale of 30–50 tonnes are already being planned to push through.
A Signal in the Gamma Rays: The 2025 Tokyo Result
Against this backdrop of persistent non-detection, a result published in late 2025 by Tomonori Totani at the University of Tokyo attracted significant attention. Analysing data from NASA's Fermi Gamma-ray Space Telescope, Totani identified a high-energy gamma-ray signal at roughly 20 GeV that fits the expected signature of dark matter particles annihilating each other in the galactic halo — the extended dark matter cloud surrounding our galaxy. The signal matches what physicists call a "halo-like excess" of diffuse gamma-ray emission, which is precisely the pattern that WIMP annihilation would produce.
This is not a confirmed detection. Gamma-ray excesses have appeared in Fermi data before — most notably the Galactic Centre Excess, which generated enormous excitement around 2014 before alternative explanations involving millisecond pulsars gained traction. Totani's signal would require confirmation from independent observations, particularly from regions of the sky known to contain high dark matter densities, such as dwarf satellite galaxies orbiting the Milky Way. If the same gamma-ray signature appears in multiple dark-matter-rich regions with consistent intensity, the case would become substantially stronger. As of April 2025, that confirmation has not arrived — but the signal has not been dismissed either.
The Other Darkness: What Dark Energy Is and Isn't
Dark energy is a different kind of problem. Dark matter, at least, is matter — something with mass that gravitationally attracts other matter and clusters into halos around galaxies. Dark energy is the opposite: it is distributed uniformly throughout space, and rather than pulling things together, it pushes them apart. It is the reason the universe is not merely expanding but accelerating — growing faster and faster with each passing billion years.
The simplest description of dark energy is Einstein's cosmological constant — a fixed, inherent energy density of empty space itself. Einstein originally introduced this term to produce a static universe, then removed it when Hubble's observations showed the universe was expanding, calling it his greatest blunder. After 1998, when observations of distant supernovae showed the expansion was accelerating, the constant was rehabilitated. Theoretically, it represents vacuum energy: the ground-state energy of quantum fields that permeate all of space.
The theoretical problem with this is severe. When physicists calculate how much vacuum energy quantum field theory predicts, they get a number. When they compare that number to the observed cosmological constant, the discrepancy is roughly 10120 — a factor of one followed by 120 zeros. This is, by most reckonings, the worst numerical prediction in the history of physics. The universe has vastly, inexplicably less dark energy than quantum mechanics says it should. We don't know why. This discrepancy, called the cosmological constant problem, is one of the deepest unresolved puzzles in all of theoretical physics.
1998: The Year Physics Got a Shock It Hasn't Recovered From
In the mid-1990s, two independent teams — the Supernova Cosmology Project led by Saul Perlmutter, and the High-Z Supernova Search Team led by Brian Schmidt and Adam Riess — were using Type Ia supernovae as cosmological measuring rods. Type Ia supernovae are useful for this because they all explode with roughly the same intrinsic brightness, which means their apparent brightness in the sky tells you how far away they are. By measuring distant supernovae, both teams were trying to determine whether the universe's expansion was slowing down under the influence of gravity.
In 1998, both teams published, independently, the same finding: the expansion was not slowing down. It was speeding up. The distant supernovae were fainter than expected — meaning they were farther away than a decelerating universe would have placed them. The universe's expansion had been accelerating for the past several billion years, driven by some form of energy no one could identify. Perlmutter, Schmidt, and Riess shared the 2011 Nobel Prize in Physics for the discovery.
The discovery was not just surprising. It was philosophically destabilising. Physicists had expected to measure the rate at which gravity was slowing down the universe. Instead they had found that gravity was losing — that something else, something with no prior theoretical justification and no physical explanation, was pushing the cosmos apart with increasing force. The 1998 result fundamentally changed what physicists thought they knew about the content and fate of the universe. It also gave rise to a question that has been growing more urgent ever since: is that force actually constant?
DESI 2025: The Cosmological Constant May Not Be Constant
The Dark Energy Spectroscopic Instrument, mounted on the Mayall 4-metre telescope at Kitt Peak Observatory in Arizona, has mapped more than 14 million galaxies across most of the universe's observable history. By measuring the slight clustering patterns left in galaxy distributions by sound waves from the early universe — a technique called baryon acoustic oscillations — DESI can track how fast the universe was expanding at different periods in its past.
In March 2025, DESI released its second major dataset — three years of observations compared to the 13-month sample used in April 2024. The result, published in coordination with independent analyses from the Dark Energy Survey (DES), the Atacama Cosmology Telescope (ACT), and a quick data release from Europe's Euclid satellite, showed a preference for evolving dark energy — dark energy whose density or strength changes over time — at 4.2 sigma confidence. In physics, 5 sigma is the conventional threshold for a discovery. At 4.2 sigma, this is strong evidence, not yet a confirmed result. But it has not weakened as more data arrived. It has strengthened.
The specific pattern DESI is finding suggests that dark energy was stronger in the past than it is now — that the force pushing the universe apart has been weakening over cosmic time. This is difficult to explain with a simple cosmological constant, which by definition does not change. If the signal is real, it means dark energy is a dynamic entity — a field that evolves, possibly a form of energy called quintessence, or something even more exotic. Josh Frieman, an architect of the original Dark Energy Survey, put it plainly: for two decades the data consistently pointed to a constant. Now, for the first time in over twenty years, they may not be.
DESI Dark Energy Timeline
| Date |
Finding |
Significance |
| Apr 2024 |
First DESI data release hints at evolving dark energy (13 months data) |
3.9 sigma |
| Mar 2025 |
DESI DR2 — 3-year data, 14 million galaxies — confirmed across DES, ACT, Euclid |
4.2 sigma |
| Sep 2025 |
University of Chicago combined multi-dataset analysis confirms dynamic dark energy over cosmological constant |
Multi-dataset |
Big Crunch, Big Rip, Big Freeze: Three Possible Ends
The nature of dark energy determines the fate of the universe, which is not a small thing to have hanging on an unresolved question. There are three broad scenarios, and which one applies depends critically on whether dark energy is constant, weakening, or strengthening over time.
If dark energy is a constant cosmological constant, the universe expands forever at an accelerating rate. Galaxies beyond the Local Group recede past our cosmic horizon, becoming permanently unreachable. Stars exhaust their fuel, black holes eventually evaporate through Hawking radiation, and the universe settles into a cold, dark, diffuse equilibrium — the Big Freeze. On timescales of 10100 years or more, even the black holes are gone and the universe is an expanding void of elementary particles slowly cooling toward absolute zero.
If dark energy is weakening — which is what the current DESI data tentatively suggests — the universe may eventually stop accelerating and begin to decelerate again. If the cosmological constant is actually negative, as Cornell physicist Henry Tye argued in a paper published in early 2026, the expansion would eventually reverse. The universe would begin to contract, and some tens of billions of years from now, everything would collapse back into a singularity — the Big Crunch. Tye's model, based on combining DESI data with his analysis of the cosmological constant, estimates this would occur in approximately 20 billion years.
If dark energy is strengthening — a scenario called phantom dark energy, where the equation of state parameter w drops below −1 — the accelerating expansion would become so powerful that it eventually overcomes not just gravity between galaxies, but the electromagnetic force holding atoms together, and ultimately the strong nuclear force holding atomic nuclei together. Everything, from galaxy clusters to individual protons, would be torn apart in a Big Rip. Current data does not strongly favour this scenario, but it cannot be ruled out.
What We Still Don't Know
We do not know what dark matter is. Thirty years of increasingly sensitive searches have narrowed the possibilities but delivered no confirmed particle. The WIMP paradigm — once so compelling that physicists spoke of a "WIMP miracle" — has been cornered. Something else, an axion or sterile neutrino or something not yet theorised, may be responsible for 27 percent of the universe's total content.
We do not know what dark energy is. The simplest answer — vacuum energy, the cosmological constant — produces the worst numerical prediction in the history of science. The second simplest answer — a dynamic field called quintessence — is gaining observational support from DESI but is not yet confirmed. The most unsettling possibility, that Einstein's general relativity is itself incomplete at cosmological scales, remains a live option.
We do not know whether dark matter and dark energy are connected. Some theoretical proposals posit a "dark sector" — a parallel family of particles and forces that interact with ordinary matter only gravitationally, with dark matter and dark energy as separate faces of the same underlying physics. These models are speculative, but they are not more speculative than the models that correctly predicted the Higgs boson, which experimentalists found 48 years after it was theoretically proposed.
What is not speculative is the scale of the problem. Of everything in the universe — all the matter and energy that makes the cosmos what it is — only 5 percent has been directly identified. The rest is not simply unknown in the sense that a distant asteroid is unknown, waiting to be measured. It is unknown in the sense that we do not know what category of thing it is. The 95 percent is not a gap in our map. It is a sign that the map itself is wrong.
Next in the Series
Episode 03: The Event Horizon
At the edge of a black hole, time slows to a crawl and information may cease to exist. Stephen Hawking and Leonard Susskind spent decades arguing about what happens when something crosses the point of no return — and the answer may rewrite the rules of physics.
Disclaimer: This article is written for general educational purposes. All scientific claims are sourced from peer-reviewed literature or established scientific institutions and are cited below. The DESI dark energy results (4.2 sigma, March 2025), the LZ non-detection (December 2025), and the University of Tokyo gamma-ray excess (2025) are active research findings — significant and credible, but not yet at the conventional 5-sigma threshold for a physics discovery. They are presented here as the current state of evidence, not established fact. Cosmology is a rapidly evolving field; conclusions described in this episode represent the frontier as of April 2025. All sources are publicly available and legally accessible.
References
- LUX-ZEPLIN (LZ) Collaboration. (2025). Extended dark matter search results, 417 live days (March 2023–April 2025). Lawrence Berkeley National Laboratory. → lbl.gov/news/lz-2025
- DESI Collaboration. (2025). Second data release — 3-year baryon acoustic oscillation survey, dark energy equation of state. Nature Astronomy commentary: → nature.com/articles/s41550-025-02549-z
- Frieman, J. & Shajib, A. (2025). Reconsidering the cosmological constant — combined dark energy analysis. Physical Review D. University of Chicago. → uchicago.edu/reconsidering-cosmological-constant
- Totani, T. (2025). 20 GeV halo-like excess of the Galactic diffuse emission and implications for dark matter annihilation. Journal of Cosmology and Astroparticle Physics. University of Tokyo. → sciencedaily.com (summary)
- Son, J. et al. (2025). Strong progenitor age-bias in supernova cosmology — alignment with DESI BAO. Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/staf1685. → ras.ac.uk
- Clowe, D. et al. (2006). A direct empirical proof of the existence of dark matter — Bullet Cluster. The Astrophysical Journal Letters, 648, L109–L113. → doi.org/10.1086/508162
- Riess, A.G. et al. (1998). Observational evidence from supernovae for an accelerating universe. The Astronomical Journal, 116(3), 1009–1038. → doi.org/10.1086/300499
- NASA Science. What is Dark Energy? → science.nasa.gov/dark-energy
- Particle Data Group, Navas et al. (2024). Review of Particle Physics — Dark Matter chapter. Physical Review D, 110, 030001. → pdg.lbl.gov (Dark Matter review, 2024)
The cosmos is not merely a vast, empty void; it is a laboratory where the fundamental laws of reality are pushed to their breaking point. When we look at the night sky, we aren't just seeing stars—we are looking at history. We are seeing light that has travelled for billions of years, witnessing the expansion of space-time, and grappling with the fact that 95% of everything out there is completely invisible to us.
In this section of Decoding Curiosity, we explore the outer reaches of understanding—from the faint echoes of the Big Bang to the crushing silence of black holes. This isn't just a collection of facts about planets and galaxies; it is a journey into the mechanics of existence, where mathematics and curiosity meet the infinite.
Explore all our articles on the universe here: Space
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