In 1933, Swiss astronomer Fritz Zwicky was studying the Coma Cluster — a collection of over 1,000 galaxies roughly 320 million light-years from Earth. He measured the velocities of individual galaxies within the cluster and applied the virial theorem: a cluster of objects orbiting in a gravitational system should have a specific relationship between their kinetic energy (how fast they move) and the gravitational potential energy (how much mass is holding them together).
The galaxies in the Coma Cluster were moving far too fast. If the cluster's gravity came only from the visible stars and gas, it would not have nearly enough mass to hold the galaxies together at those velocities. The cluster should be flying apart. It was not flying apart.
Zwicky concluded that there must be enormous amounts of invisible "dunkle Materie" — dark matter — providing the gravitational glue the visible matter could not. He estimated the cluster contained 400 times more dark mass than luminous mass.
His conclusion was largely ignored for four decades. In the 1970s, astronomer Vera Rubin produced independent, overwhelming evidence from a completely different type of observation — and dark matter's existence became the consensus view of cosmology.
Then in 1998 came the second shock. Two independent teams studying distant supernovae expected to measure the rate at which the universe's expansion was slowing down — they were going to use supernovae as cosmic yardsticks to measure the deceleration caused by gravity. Instead, they found the expansion was speeding up. Something was pushing the universe apart — something that accounted for roughly 68% of the universe's total energy content and whose physical nature was entirely unknown.
The universe we see, touch, measure, and understand — all the atoms in all the stars in all the galaxies — constitutes just 5% of what exists.
"Not only are we not at the center of the universe, but we're not even made of the dominant stuff of the universe. We're like a thin frosting on a very large, very dark cake." — Vera Rubin, various public lectures (1990s)
Key Definitions
Dark matter — A form of matter that does not interact with electromagnetic radiation (it neither emits, absorbs, nor reflects light) but exerts gravitational effects consistent with additional mass beyond visible matter. Constitutes approximately 27% of the universe's total energy content. First inferred from galaxy cluster dynamics by Fritz Zwicky (1933); confirmed by galaxy rotation curve measurements by Vera Rubin (1970s). Physical identity unknown.
Dark energy — A form of energy permeating all space, responsible for the accelerating expansion of the universe. Constitutes approximately 68% of the universe's total energy content. Discovered in 1998 through Type Ia supernova distance measurements (Saul Perlmutter, Brian Schmidt, Adam Riess; Nobel Prize 2011). May be the cosmological constant (vacuum energy), a dynamic field (quintessence), or something else. Physical origin completely unknown.
Baryonic matter — Ordinary matter made of protons, neutrons, and electrons — atoms, molecules, gas, dust, stars, planets, and people. Constitutes approximately 5% of the universe's total energy content. All visible matter is baryonic.
Non-baryonic matter — Matter not made of protons and neutrons. Dark matter must be non-baryonic: Big Bang nucleosynthesis constrains baryonic matter to about 5% of the universe's energy content, far less than the 27% inferred from gravitational observations.
Rotation curve — A plot of the orbital velocities of stars and gas in a galaxy versus their distance from the galactic center. Newtonian gravity predicts that orbital velocity should decline with distance from the center (as in the solar system). Observed rotation curves are flat — velocity does not decline with distance — which requires additional mass (dark matter) in an extended halo beyond the visible galaxy.
Gravitational lensing — The bending of light from background objects by the gravity of foreground masses. Einstein's general relativity predicts this bending precisely; the amount of bending measures the total gravitating mass, visible and invisible. Dark matter halos around galaxies and galaxy clusters produce more lensing than visible matter alone would predict.
WIMP (Weakly Interacting Massive Particle) — The leading dark matter candidate class: hypothetical particles with masses roughly 10 to 1,000 times the proton mass that interact via the weak nuclear force and gravity but not electromagnetism or the strong force. WIMPs arise naturally in supersymmetric extensions of the Standard Model of particle physics. Extensive searches have so far found no WIMPs, placing increasingly tight constraints on their properties.
Axion — A hypothetical light particle proposed in 1977 by Roberto Peccei and Helen Quinn to solve the strong CP problem in quantum chromodynamics. Axions could have the right properties to be dark matter if their mass falls in the range of 10⁻⁶ to 10⁻³ eV. Experimental searches are ongoing.
Cosmological constant (Λ) — A term Einstein added to his field equations in 1917 to produce a static universe, representing an energy density of empty space. After Hubble's discovery of expansion, Einstein reportedly called it his "greatest blunder." Revived to explain dark energy after 1998 — the cosmological constant provides the simplest explanation for accelerating expansion. The observed value is approximately 10⁻¹²² times the value predicted by quantum field theory, a discrepancy of 122 orders of magnitude — the worst prediction in physics, known as the cosmological constant problem.
Quintessence — A proposed alternative to the cosmological constant: a dynamic scalar field whose energy density evolves over time, producing dark energy that may have varied throughout cosmic history. Unlike the cosmological constant (fixed vacuum energy), quintessence predicts slight variations in the dark energy density over time that future observations may be able to detect.
Lambda-CDM model — The standard model of cosmology: Lambda (Λ) refers to the cosmological constant (dark energy) and CDM refers to Cold Dark Matter (dark matter consisting of slow-moving, massive particles). Lambda-CDM fits virtually all cosmological observations with just six parameters and is the current consensus framework.
What We Know (and Don't Know): A Summary
| Dark Matter | Dark Energy | |
|---|---|---|
| Share of universe | ~27% | ~68% |
| What it does | Gravitational glue holding galaxies and clusters together | Drives accelerating expansion of universe |
| How discovered | Galaxy rotation curves (Rubin 1970s); cluster dynamics (Zwicky 1933) | Type Ia supernova distances (1998) |
| Direct detection | None yet — extensive searches ongoing | Not directly detectable — inferred from expansion |
| Best-supported candidate | WIMPs or axions | Cosmological constant (vacuum energy) |
| Biggest theoretical problem | Why haven't we detected WIMPs after decades of searching? | Cosmological constant 10¹²² times smaller than QFT predicts |
| Nobel Prize | No — despite decades of candidate searches | Yes — Perlmutter, Schmidt, Riess (2011) |
The Evidence for Dark Matter
1. Galaxy Rotation Curves
Vera Rubin and collaborator Kent Ford spent the 1970s measuring the rotational velocities of stars at different distances from the centers of spiral galaxies. The prediction from Newtonian gravity was clear: stars at the outer edges of galaxies should orbit more slowly than those closer to the mass-concentrated center — just as Neptune orbits the Sun more slowly than Mercury.
The observations contradicted this prediction completely. Stars at the outer edges of galaxies orbit at roughly the same speed as stars near the center — flat rotation curves instead of the predicted declining ones. This requires the gravitational force on outer stars to be much larger than visible matter produces.
The explanation: galaxies are embedded in vast, roughly spherical halos of dark matter extending far beyond the visible disk. This dark halo's gravitational pull supplements the visible matter's pull, keeping outer stars moving fast enough to remain in orbit.
"I had no expectation that I would find what I found. I was looking for evidence of a simple, Keplerian decline. What I found was flat rotation curves. The data told a different story than we expected." — Vera Rubin, interview (1996)
Rubin's measurements, combined with Zwicky's earlier cluster work and subsequent observations from radio astronomy (hydrogen gas rotation curves extending even further out than visible stars), established dark matter as the consensus explanation by the early 1980s.
2. The Bullet Cluster: Dark Matter Directly Imaged
The most direct evidence for dark matter comes from the Bullet Cluster (1E 0657-558), a pair of galaxy clusters that collided roughly 150 million years ago. The collision provides a natural experiment.
When two galaxy clusters collide:
- Visible gas (which makes up most of the visible matter in clusters) interacts electromagnetically, heating to millions of degrees and slowing dramatically — it piles up in the collision zone
- Dark matter (if it exists and interacts only gravitationally) passes through without slowing, continuing on its original trajectory
- Stars (also primarily interacting gravitationally) pass through largely unimpeded
Using X-ray observations (detecting the hot visible gas) and gravitational lensing maps (detecting total mass), astronomers can compare where the visible gas is versus where the total mass is.
In the Bullet Cluster, the visible gas is concentrated at the center of the collision. The gravitational mass (detected by lensing) is distributed on either side of the collision — coinciding with the positions of the galaxies and stars, not with the gas. Dark matter clearly separated from the visible gas during the collision, moving through rather than interacting.
This is direct visual evidence that most of the cluster's mass is non-baryonic and non-interacting — dark matter.
3. Large-Scale Structure Formation
Computer simulations of the universe's evolution from the tiny CMB fluctuations to the present-day cosmic web — the filaments, voids, and clusters of galaxies — can only match observations when dark matter is included.
Without dark matter, gravity from visible matter alone cannot produce the large-scale structure we observe on the correct timescale. Dark matter's gravity seeded the gravitational collapse that pulled gas into the first galaxies. The visible universe is, in a real sense, a map drawn in lit places within a pre-existing dark matter skeleton.
The Evidence for Dark Energy
The Supernova Discovery
In 1998, two independent teams — the Supernova Cosmology Project (Saul Perlmutter) and the High-Z Supernova Search Team (Brian Schmidt and Adam Riess) — were using Type Ia supernovae as "standard candles" to measure cosmic distances.
Type Ia supernovae have a nearly constant intrinsic brightness — they explode in a consistent way that allows astronomers to calculate how bright they should appear, and compare to how bright they actually appear. The difference reveals their distance. By measuring distances to supernovae at various redshifts, they could map the history of the universe's expansion.
Both teams expected to find a decelerating expansion — gravity should be slowing the universe down. Both found instead that distant supernovae were dimmer than expected, meaning they were farther away than deceleration would predict. The universe's expansion was accelerating.
Something was pushing the universe apart, overcoming gravity on the largest scales. Something with negative pressure — the defining property of dark energy — fills all of space and grows more powerful as the universe expands.
"I was so shocked that I thought I had made an error. I went back and checked everything. The data kept saying the same thing: the universe is accelerating. I had to accept it." — Adam Riess, Nobel Lecture (2011)
Perlmutter, Schmidt, and Riess were awarded the Nobel Prize in Physics in 2011 for this discovery.
What Dark Matter and Dark Energy Might Be
Dark Matter Candidates
WIMPs remain the most theoretically motivated candidates. They arise naturally in supersymmetric extensions of the Standard Model, have the right properties to have been produced in the Big Bang in the correct quantities (the "WIMP miracle"), and could be detected in underground detectors, at particle colliders, or through annihilation products in space. Decades of searching have not found them, significantly constraining their properties but not ruling them out.
Axions have gained attention as WIMP searches have remained empty-handed. They are extremely light (much lighter than WIMPs), arise from well-motivated theoretical considerations, and could fill the universe in vast quantities. Experiments including ADMX (Axion Dark Matter Experiment) are searching for them using resonant microwave cavities in strong magnetic fields.
Primordial black holes — black holes formed in the early universe, before the formation of stars — were revived as candidates after gravitational wave detections showed more black holes than expected. Constraints from various observations limit the fraction of dark matter they can constitute, but they remain possible contributors.
Dark Energy Candidates
Cosmological constant (vacuum energy): The simplest explanation — empty space has an inherent energy density. Fits all current observations perfectly. The theoretical problem: quantum field theory predicts vacuum energy 10¹²² times larger than observed. Why the cosmological constant has such a tiny, specific value is completely unexplained.
Quintessence: A dynamic scalar field whose energy density varies with time. Future precision measurements of dark energy's behavior — whether it varies over cosmic time — could distinguish quintessence from the cosmological constant.
Modified gravity: Perhaps dark energy and dark matter are artifacts of using the wrong theory of gravity on large scales. Modified gravity theories (MOND, f(R) gravity, etc.) attempt to explain these observations without introducing new forms of matter and energy. They have not succeeded in matching all observations simultaneously.
The Deepest Unsolved Problem in Physics
The cosmological constant problem — why is the observed dark energy density 10¹²² times smaller than quantum field theory predicts? — is often described as the worst prediction in physics. The discrepancy between theory and observation is larger than for any other physical quantity.
Proposed resolutions include the anthropic principle (universes with larger cosmological constants don't allow galaxies and stars to form; we necessarily observe a universe compatible with our existence), supersymmetry cancellations (which might reduce the predicted value, though not by enough), and new theoretical frameworks not yet developed.
The two great unsolved problems in fundamental physics — quantum gravity (reconciling general relativity with quantum mechanics) and the cosmological constant problem — are closely related. Solving one may illuminate the other.
We live in a universe that is 95% mystery — and the mystery, precisely measured, is one of the greatest achievements of modern science.
For related concepts, see how the universe began, how gravity works, and what is quantum mechanics.
References
- Zwicky, F. (1933). Die Rotverschiebung von extragalaktischen Nebeln. Helvetica Physica Acta, 6, 110–127. (English translation in General Relativity and Gravitation, 41, 207–224, 2009.)
- Rubin, V., & Ford, W. K. (1970). Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions. Astrophysical Journal, 159, 379–403. https://doi.org/10.1086/150317
- Clowe, D., Bradač, M., Gonzalez, A. H., Markevitch, M., Randall, S. W., Jones, C., & Zaritsky, D. (2006). A Direct Empirical Proof of the Existence of Dark Matter. Astrophysical Journal Letters, 648, L109–L113. https://doi.org/10.1086/508162
- Perlmutter, S., et al. (1999). Measurements of Omega and Lambda from 42 High-Redshift Supernovae. Astrophysical Journal, 517, 565–586. https://doi.org/10.1086/307221
- Riess, A. G., et al. (1998). Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant. Astronomical Journal, 116, 1009–1038. https://doi.org/10.1086/300499
- Planck Collaboration. (2020). Planck 2018 Results: Cosmological Parameters. Astronomy & Astrophysics, 641, A6. https://doi.org/10.1051/0004-6361/201833910
- Carroll, S. M. (2001). The Cosmological Constant. Living Reviews in Relativity, 4, 1. https://doi.org/10.12942/lrr-2001-1
- Bertone, G., Hooper, D., & Silk, J. (2005). Particle Dark Matter: Evidence, Candidates and Constraints. Physics Reports, 405(5–6), 279–390. https://doi.org/10.1016/j.physrep.2004.08.031
Frequently Asked Questions
What is dark matter?
Matter that doesn't interact with light but exerts gravity, accounting for ~27% of the universe's energy content. Its existence is confirmed by galaxy rotation curves, gravitational lensing, and the Bullet Cluster collision — but its physical identity remains unknown.
What is dark energy?
An energy permeating all space that drives the universe's accelerating expansion, accounting for ~68% of total energy content. Discovered in 1998 via supernova distances; the simplest explanation is Einstein's cosmological constant (vacuum energy), but its physical origin is completely unknown.
How do we know dark matter exists if we can't see it?
Four independent lines of evidence: flat galaxy rotation curves (outer stars orbit too fast), gravitational lensing (more bending than visible mass predicts), the Bullet Cluster (dark matter separated from gas during a cluster collision), and structure formation (the cosmic web requires dark matter to form on the observed timescale).
Could dark matter just be undetected ordinary matter?
No. Big Bang nucleosynthesis constrains ordinary matter to ~5% of the universe — matching observed hydrogen/helium abundances exactly. Dark matter must be non-baryonic. Leading candidates are exotic particles: WIMPs, axions, or sterile neutrinos.
Why is dark energy sometimes called the cosmological constant?
Einstein's cosmological constant (Λ) — the energy density of empty space — is the simplest explanation for dark energy. The observed value matches observations, but is 10¹²² times smaller than quantum field theory predicts, the largest discrepancy between theory and observation in all of physics.
What happens to dark energy and dark matter in the future?
If the cosmological constant holds, expansion accelerates forever — stars burn out, galaxies isolate, black holes evaporate, heat death over ~10¹⁰⁰ years. If dark energy grows stronger over time, a 'Big Rip' could tear apart matter at a specific future date.