Roughly 13.8 billion years ago, the universe began. Matter formed, gravity pulled it together, and over billions of years stars, galaxies, and galaxy clusters assembled from the wreckage of the Big Bang. The expectation — the sensible, physically motivated expectation — was that this process would eventually slow down. Gravity should be putting the brakes on expansion. The universe should be decelerating.
In 1998 two independent teams of astronomers — one led by Saul Perlmutter, one by Brian Schmidt and Adam Riess — were measuring the distances to distant supernovae to settle this question once and for all. They expected to measure the deceleration. Instead, they found the opposite. The universe is not slowing down. It is speeding up. Galaxies are accelerating away from each other, and the further away they are, the faster they recede.
Something is pushing space apart. Something with negative pressure — a kind of anti-gravity that operates at cosmic scales. We have no idea what it is. We have given it a name: dark energy. And it accounts for approximately 68% of everything in the observable universe.
This is the most important unsolved problem in cosmology. Here is everything we know — and everything we do not.
What Is Dark Energy? The Simple Explanation
Dark energy is the name physicists give to whatever is causing the accelerated expansion of the universe. It is not dark in the sense of being black or absorbing light — it is dark in the sense of being completely invisible and undetectable by any instrument we have built. It interacts with matter and radiation so weakly that it leaves no trace in any laboratory experiment. We know it exists only because the universe behaves as though it does.
Think of space itself as a fabric. The Big Bang stretched that fabric outward. Gravity — matter attracting matter — tries to pull the fabric back in. For the first several billion years of cosmic history, these two tendencies roughly balanced. Then, approximately five billion years ago, something changed. The expansion began to accelerate. Whatever was causing it had grown stronger than gravity on the largest scales, and the stretching of space began to win.
That something is dark energy.
What makes dark energy particularly strange is its relationship with space. Unlike matter or radiation, which become diluted as space expands — their density decreasing as the volume they occupy increases — dark energy appears to maintain constant density as space expands. More space means more dark energy. This property, called negative pressure in physics, is what drives the acceleration: as the universe grows, dark energy does not thin out, so its influence relative to gravity continuously increases.
Dark Energy vs Dark Matter: What Is the Difference?
Dark energy and dark matter are frequently confused because they share the word “dark” — but they are entirely different phenomena with nothing in common except our ignorance of their nature.
| Property | Dark Matter | Dark Energy |
|---|---|---|
| What it does | Pulls matter together — acts as extra gravity | Pushes space apart — drives accelerated expansion |
| Share of universe | ~27% | ~68% |
| Where it concentrates | Halos around galaxies and clusters | Uniformly distributed throughout all space |
| Effect on structure | Helps galaxies and clusters form | Prevents new large structures from forming |
| Detection method | Gravitational lensing, galaxy rotation curves | Supernova distance measurements, CMB, BAO |
| Discovery | 1933 (Zwicky), confirmed 1970s (Rubin) | 1998 (Perlmutter, Schmidt, Riess) |
Dark matter causes galaxies to rotate faster than visible matter alone can explain — it is the gravitational scaffolding that holds cosmic structure together. Dark energy does the opposite — it tears structure apart on the largest scales, preventing galaxy clusters from growing and eventually, if it continues accelerating, potentially dissolving all bound structures in the far future. For a full exploration of dark matter — what it is, how we know it exists, and what it might be made of — see our article on dark matter explained.
How Do We Know Dark Energy Exists? The Evidence
Dark energy is inferred from several independent lines of evidence, each of which points to the same conclusion. The convergence of independent methods is what gives cosmologists confidence in its existence despite its complete invisibility.
Type Ia Supernovae. Type Ia supernovae — the thermonuclear explosions of white dwarf stars — all explode at roughly the same intrinsic brightness, making them reliable distance markers. By comparing how bright they appear from Earth against how bright they should be, astronomers can calculate exactly how far away they are and how fast the universe was expanding when the light left them. The 1998 measurements showed that distant supernovae were dimmer than expected — meaning they were further away than a decelerating universe would place them. The expansion was accelerating. This discovery earned Perlmutter, Schmidt, and Riess the 2011 Nobel Prize in Physics.
The Cosmic Microwave Background. The CMB — the thermal afterglow of the Big Bang, mapped in extraordinary detail by the Planck satellite — encodes the geometry of the universe. A universe with the observed CMB pattern must be spatially flat and must contain a specific total energy density. Ordinary matter and dark matter together account for only about 32% of that density. The remaining 68% must come from something else — dark energy.
Baryon Acoustic Oscillations. BAO are patterns in the distribution of galaxies — regular fluctuations in galaxy density at a specific scale, imprinted by sound waves in the early universe. By measuring the BAO scale at different cosmic epochs using large galaxy surveys, astronomers can track how the universe’s expansion history has evolved. The measurements are consistent with accelerating expansion driven by dark energy.
Large-Scale Structure. The rate at which galaxy clusters grow over cosmic time is sensitive to dark energy. Dark energy suppresses the growth of structure — it prevents gravity from assembling new large clusters. Measurements of cluster counts as a function of redshift are consistent with the presence of dark energy at the level indicated by other methods.
The Cosmological Constant: Einstein’s Greatest Blunder — or His Greatest Insight?
The most mathematically natural description of dark energy is Einstein’s cosmological constant — a term he introduced into his field equations in 1917, represented by the Greek letter Lambda (Λ). Einstein added it to make his equations describe a static universe, which seemed physically reasonable at the time. When Hubble showed in 1929 that galaxies were receding, Einstein abandoned the cosmological constant, reportedly calling it his “greatest blunder.”
In 1998, the cosmological constant came back from the dead. The accelerated expansion that the supernova teams discovered is exactly consistent with a positive cosmological constant — a fixed, uniform energy density of empty space. The standard model of cosmology, Lambda-CDM, adopts the cosmological constant as its description of dark energy.
The problem is that quantum field theory predicts a value for the vacuum energy — the energy of empty space from quantum fluctuations — that is approximately 10 to the power of 120 times larger than the cosmological constant that observations require. This discrepancy — the largest in the history of science between a theoretical prediction and an observation — is called the cosmological constant problem. It is one of the deepest unsolved problems in theoretical physics.
Is Dark Energy Constant? What New Data Suggests
The Lambda-CDM model treats dark energy as a fixed cosmological constant — the same everywhere, always, unchanging. But recent data has introduced significant uncertainty about this assumption.
The Dark Energy Spectroscopic Instrument, or DESI, is a survey instrument on the Mayall Telescope at Kitt Peak Observatory in Arizona that is mapping the three-dimensional positions of tens of millions of galaxies. In April 2024, the DESI collaboration released its first-year results — and they suggested, at a statistical significance just above the threshold that physicists consider interesting, that dark energy may not be constant. The data hinted that the density of dark energy may have been higher in the past and is decreasing over time — a behaviour inconsistent with a simple cosmological constant.
DESI’s second-year results, released in March 2025, strengthened this signal. The data now suggests, at roughly 3 sigma significance, that dark energy evolves — that it is not a fixed property of empty space but something dynamic, changing over cosmic time. Three sigma is not the five-sigma threshold physicists require to claim a discovery, but it is significant enough that the cosmological community is taking it seriously.
If dark energy is not constant but evolves, the cosmological constant is not the right description. What is? Several alternatives have been proposed.
Theories of Dark Energy: What It Might Actually Be
If dark energy is not simply the energy of the vacuum — not a cosmological constant — several alternative explanations have been developed.
Quintessence is a dynamic scalar field — a quantum field permeating all of space — whose energy density evolves over time. Unlike the cosmological constant, quintessence can change its value across cosmic history, potentially explaining the DESI results. It introduces a new fundamental field into physics, analogous to the Higgs field, but one that has so far not been detected by any other means.
Modified gravity theories propose that dark energy is not a substance at all but a signal that Einstein’s general theory of relativity breaks down on cosmological scales. If gravity behaves differently than general relativity predicts over the largest distances, the apparent accelerated expansion might be an artefact of the wrong gravitational theory rather than evidence for a new form of energy. Several modified gravity models make specific predictions that ongoing surveys are testing.
Microscopic wormholes — a proposal published in Physical Review D in early 2025 — suggest that dark energy might arise from the continuous creation and annihilation of quantum-scale wormholes in the vacuum of space, contributing an effective energy density that behaves like dark energy and naturally evolves over time. The proposal fits the DESI data better than a fixed cosmological constant. For a full exploration of this idea and the physics behind it, see our article on the wormhole solution: could microscopic wormholes be driving the expansion of the universe?
Phantom energy is a theoretical form of dark energy whose density increases over time, leading to an eventual scenario called the Big Rip — in which the accelerating expansion eventually tears apart galaxy clusters, then individual galaxies, then solar systems, then planets, then atoms themselves. Current data does not strongly favour phantom energy, but it has not been ruled out.
The Hubble Tension: Dark Energy’s Role in Cosmology’s Biggest Controversy
One of the most active debates in cosmology concerns a discrepancy called the Hubble tension — a significant disagreement between two independent measurements of the Hubble constant, the number that describes the current rate of the universe’s expansion.
Measurements of the CMB from the Planck satellite give a Hubble constant of approximately 67.4 kilometres per second per megaparsec. Measurements of the local universe — using Cepheid variable stars and Type Ia supernovae as distance ladders — give approximately 73 kilometres per second per megaparsec. The difference is about 8%, which is five to six times larger than the combined measurement uncertainties. Something is wrong.
One possibility is that dark energy is not constant — that it was different in the early universe than it is now, in a way that affects the early-universe measurement but not the late-universe one. The DESI results, which suggest evolving dark energy, are consistent with this possibility and may be pointing toward the resolution of the Hubble tension. If confirmed, this would be one of the most significant developments in cosmology in decades, simultaneously solving two problems with a single mechanism.
The Fate of the Universe: What Dark Energy Means for the Cosmic Future
Dark energy’s ultimate significance may be the fate it is engineering for the universe. Three broad scenarios exist, depending on what dark energy actually is and how it evolves.
If dark energy is the cosmological constant — fixed and unchanging — the universe will expand forever at an accelerating rate. Galaxies beyond our local group will eventually recede faster than light and disappear from our observable horizon. The Milky Way and its local neighbours will merge into a single large elliptical galaxy, which will then exist in increasing isolation as the rest of the universe accelerates away. Eventually, over trillions of years, stars will burn out, black holes will evaporate through Hawking radiation, and the universe will approach a cold, dark, and maximally disordered state — the heat death of the universe.
If dark energy is phantom energy — growing stronger over time — the Big Rip awaits. In this scenario, the accelerating expansion eventually overcomes every force in nature. Galaxy clusters are torn apart, then galaxies, then solar systems, then planets, then molecules, then atoms. The universe ends not with a whimper but with a violent shredding of everything that exists.
If dark energy is weakening — if the DESI signal reflects a genuine decrease in dark energy density over time — the expansion might eventually slow, stop, and reverse. The universe could collapse back on itself in a Big Crunch, or oscillate through repeated expansions and contractions.
We do not currently know which of these futures awaits. The answer depends on the nature of dark energy — the question we cannot yet answer.
Current and Future Missions Hunting Dark Energy
Several major observational programmes are currently targeting dark energy with the goal of measuring its properties precisely enough to distinguish between competing theories.
DESI (Dark Energy Spectroscopic Instrument) will complete a five-year survey mapping approximately 40 million galaxies and quasars, providing the most precise measurement of baryon acoustic oscillations ever achieved. Its preliminary results suggesting evolving dark energy will be either confirmed or refuted by its final dataset.
Euclid, the European Space Agency’s space telescope launched in 2023, is conducting a weak gravitational lensing survey of the entire extragalactic sky, mapping the distribution of dark matter and dark energy across cosmic time with unprecedented precision. For more on gravitational lensing as a cosmological tool, see our article on gravitational lensing: how the universe uses gravity as a telescope.
The Vera C. Rubin Observatory, whose Legacy Survey of Space and Time began operations in 2025, will photograph the entire southern sky every few nights for ten years, generating an unprecedented dataset for supernova cosmology, weak lensing, and large-scale structure measurements.
The Nancy Grace Roman Space Telescope, due to launch in the late 2020s, will conduct a wide-field infrared survey specifically designed to measure dark energy through multiple independent methods simultaneously.
The convergence of these datasets over the next decade should determine whether dark energy is constant or evolving — and if evolving, in what way. The answer will either confirm the cosmological constant as nature’s choice, or open an entirely new chapter in fundamental physics.
The Deepest Question: Why Is the Universe Accelerating at All?
Behind all the observational programmes and competing theories lies a question that current physics cannot answer: why does the universe have the specific amount of dark energy it has?
The observed cosmological constant is not zero — which would require explanation — and not the enormous value quantum field theory naively predicts — which would be catastrophic for the existence of any structure. It is a tiny positive value, finely tuned to a degree that defies conventional explanation, sitting in a narrow range that permits galaxies, stars, and ultimately life to exist. A cosmological constant much larger would have torn the universe apart before any structure could form. A cosmological constant much smaller — or negative — would have led to a rapid recollapse.
This fine-tuning is one of the deepest puzzles in physics. The leading theoretical framework for addressing it — the anthropic principle applied to a multiverse of universes with different cosmological constants — is philosophically contentious and empirically unfalsifiable. Other approaches, including dynamical dark energy models and modifications of quantum gravity, attempt to explain the value from first principles but have not yet succeeded.
Dark energy is not just an astronomical curiosity. It is a signal from nature that our deepest theories — quantum mechanics and general relativity — are incomplete in a way that matters at the largest scales. Solving it may require the same kind of conceptual revolution that general relativity represented a century ago. For a look at the quantum vacuum energy that may lie at the heart of dark energy, see our article on from nothing to everything: how the universe emerged from the quantum vacuum. And for the baryonic matter that dark energy is pushing apart, see our article on baryons: the building blocks of all matter.
Frequently Asked Questions
What is dark energy in simple terms?
Dark energy is the name for whatever is causing the universe’s expansion to accelerate. It is invisible, fills all of space uniformly, and unlike matter does not become diluted as the universe expands. It accounts for approximately 68% of the total energy content of the observable universe. We know it exists because of how the universe behaves at large scales, but we do not know what it fundamentally is.
Is dark energy the same as dark matter?
No. Dark matter and dark energy are completely different phenomena. Dark matter is an invisible form of matter that exerts gravitational attraction — it helps hold galaxies together and build cosmic structure. Dark energy is an invisible form of energy that has a repulsive effect on space — it drives the accelerated expansion of the universe and works against gravity on the largest scales.
How was dark energy discovered?
Dark energy was discovered in 1998 when two independent teams measuring the distances to distant Type Ia supernovae found that those supernovae were further away than a decelerating universe would place them. The universe was not slowing down — it was speeding up. The discovery earned Saul Perlmutter, Brian Schmidt, and Adam Riess the 2011 Nobel Prize in Physics.
Could dark energy be Einstein’s cosmological constant?
The cosmological constant — a fixed energy density of empty space that Einstein introduced in 1917 — is the simplest and most mathematically natural description of dark energy, and the standard cosmological model (Lambda-CDM) uses it. Recent DESI survey results suggest dark energy may be evolving over time rather than constant, which would mean the cosmological constant is not the correct description. This remains under active investigation.
What is the cosmological constant problem?
Quantum field theory predicts a value for the vacuum energy density — the energy of empty space — that is approximately 10 to the power of 120 times larger than the dark energy density actually observed. This discrepancy, the largest between theory and observation in the history of science, is the cosmological constant problem. It suggests something fundamental is missing from our understanding of either quantum field theory or gravity.
Will dark energy eventually destroy the universe?
If dark energy is the cosmological constant, the universe will expand forever into an increasingly cold and isolated state — the heat death. If dark energy is phantom energy that grows stronger over time, it could eventually tear apart all structures in the universe in a Big Rip. If dark energy is weakening, the expansion could eventually reverse into a Big Crunch. Current data does not definitively distinguish between these scenarios.
Further Reading
- Nobel Prize in Physics 2011 — Dark Energy Discovery
- DESI — Dark Energy Spectroscopic Instrument
- ESA — Euclid Mission
- Wikipedia — Dark Energy
- Wikipedia — Cosmological Constant Problem
- The Biggest Ideas in the Universe by Sean Carroll — the clearest popular treatment of dark energy and cosmology
Sources
- Nobel Prize in Physics 2011
- DESI Survey
- ESA Euclid Mission
- Wikipedia — Dark Energy
- Wikipedia — Cosmological Constant
- Wikipedia — Accelerating Expansion
- Wikipedia — Hubble Tension
- Web News For Us — Dark Matter Explained
- Web News For Us — The Wormhole Solution
- Web News For Us — Gravitational Lensing
- Web News For Us — From Nothing to Everything
- Web News For Us — Baryons
About the Author
Baryon is the founder and editor of Web News For Us. Driven by a deep fascination with the biggest unanswered questions in science — from quantum physics and cosmology to the nature of consciousness and the genetic code written into every living cell — he has spent years studying modern physics, biology, and the history of scientific thought. He covers Science & AI, Space, Genetics & Research, and the timeless wisdom of history’s greatest thinkers and mystics.
If you have ever looked at the night sky and felt that pull to understand what is out there or the wondered about an entire universe coiled inside your genes — you are in the right place.
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