It’s hard to believe, but the idea that the Universe was dominated not by normal matter but rather by dark matter — a novel form of non-interacting matter that’s completely distinct from protons, neutrons, and electrons — goes all the way back to 1933: nearly a full century ago. For decades, the overwhelming majority of the leading astronomers and physicists dismissed the idea as being ill-motivated and lacking sufficient evidence backing it, and it gained very little traction on both the theoretical and observational fronts throughout the 1930s, 1940s, 1950s, and 1960s. It was only with the novel results and improved instrumentation initially leveraged by Vera Rubin and Kent Ford, and then further developed by Rubin on her own, that dark matter was brought into the cosmological mainstream in 1969 and then throughout the 1970s.
So who should get the credit for discovering dark matter? Did either Fritz Zwicky, who first presented that initial 1933 evidence and even coined the term dunkle materie, which directly translates to dark matter, or Vera Rubin, with her pioneering work concerning galactic rotation in the 1970s, actually discover dark matter? Could either of them be credited with at least presenting the overwhelming evidence in favor of dark matter’s existence? Or is it unfair to say that dark matter was actually discovered by either of them, including up through and including the present day?
Although the astronomical evidence is overwhelming for dark matter’s existence, attributing dark matter’s “discovery” to any one individual misses the entire point of science, including how it’s conducted and how conclusions are reached. Here’s a context-rich history of dark matter that might surprise you in many ways.
By the time the 1930s came along, even though that was already a full 93 years ago, astronomy was actually quite advanced as a science.
- Telescope apertures had already reached 100 inches (2.54 meters, which is larger than the Hubble Space Telescope’s mirror) and a 200-inch (5.1 meter) telescope — double the diameter and with four times the light-gathering power — was already under construction.
- We had learned that the spiral and elliptical nebulae in the sky were actually galaxies unto themselves, with their own stars and matter inside, located millions of light-years beyond the Milky Way.
- We knew the properties of stars and how brightness, mass, color/temperature, and ionization were all related.
- We knew that the Universe was expanding, with light from more distant galaxies appearing systematically redshifted directly dependent on their distance from us.
- On top of that, we had even measured the expansion rate: the first determination of what we now know as the Hubble constant.
It was with this picture of the cosmos that the first hints of dark matter were discovered. In 1933, Fritz Zwicky was studying the galaxies in the Coma Cluster: a cluster of galaxies located just ~300 million light-years away. With over 1,000 identified galaxies in it today, it’s larger, richer, and more regular than the nearby Virgo Cluster (itself only ~50–60 million light-years away), and many of its galaxies are large, bright, and luminous.
Even with the instruments available to Zwicky at the time, he was able to identify dozens of individual member galaxies of the Coma Cluster, including a number of bright spirals (mostly toward the cluster’s outskirts) and giant ellipticals (mostly toward the cluster’s center). These individual galaxies all have approximately the same redshift, because at around 320 million light-years away, the entire cluster is receding from us along with the expanding Universe. When he took the average redshift of the galaxies in the cluster, he got a value that corresponded to a speed of just about 2% the speed of light. The cluster was definitely speeding away from us along with the expansion of the Universe.
But Zwicky didn’t have to settle for the average value of the redshift across many different galaxies; he was able to measure the redshift of each member galaxy that he could resolve individually. Some of them — maybe even most of them — were moving with either the average value or a value close to the average, as indicated by their redshift. But others possessed redshift values that were much higher or much lower than the average, indicating that these galaxies making up the cluster were zipping around incredibly fast inside. It was as though the individual galaxies were swarming, like a hive of bees, around and through the galaxy cluster itself.
In order for that configuration to be a stable one, there had to be an enormous amount of mass holding this galaxy cluster together: with enough gravity to keep these fast-moving galaxies from escaping from the cluster’s overall gravitational pull. Since there was no evidence that this cluster (or any similar cluster) of galaxies was flying apart, that mass must be present, even if we couldn’t see it.
Zwicky’s reasoning proceeded as follows:
- as astronomers, we know how stars work,
- and if we measure the starlight from all the galaxies in the cluster that we see, we can determine how much mass is in these galaxies and in the whole cluster,
- we also know how gravity and the expanding Universe works,
- so if we measure the average redshift of the cluster, we know how far away it is,
- and based on how fast we see these galaxies moving, there must be at least a certain amount of mass in there due to gravity.
When he compared the “mass from starlight” number with the “mass from gravitation” number, he realized that the latter number was over 400 times larger than the first number. Even if there were a small, unidentified error somewhere, he argued, this was an incredibly high magnitude of a mismatch, and no small error could account for all of it. Therefore, as a necessity, there must be much more matter out there than the normal matter that we knew of could account for. He called this unseen matter dunkle materie: directly translated as dark matter.
Zwicky was quite a gifted astronomer, but also an iconoclast who was known for agitating the mainstream. Perhaps as a result of that, his conclusions were doubted by most professionals in the field, with some of them possessing a multitude of good reasons for doing so. It wasn’t necessarily a dogmatic adherence to the consensus picture at the time, but rather it was the large cosmic unknowns that had yet to be resolved, that prevented the idea of dark matter from taking hold among the community.
Here were some of the problems with Zwicky’s conclusions that several of his contemporary astronomers raised.
- Inferring the distance to the Coma Cluster: What you measure for a distant galaxy is just a redshift and an observed brightness. If you want to know the distance and don’t have a direct measurement (which we didn’t, at the time, for any of Zwicky’s galaxies), you have to infer it from the Hubble constant. But, at the time, the Hubble constant had such an absurdly high estimate that taking its value seriously implied a ~2 billion year old Universe: a Universe less than half the age of the Earth! If that was unreliable, how could we rely on the distance estimate to the cluster itself?
- Stars are not like the Sun, on average: After measuring the cumulative light from the stars in the observed Coma Cluster galaxies, Zwicky then assumed they had the same overall mass-to-light ratio that the Sun possesses. However, the light from galaxies is dominated not by stars like our Sun, but by hotter, bluer, more massive stars. Based on the observed light that Zwicky saw, there should have actually been multiple times the amount of mass inside that he assumed; the mass-to-light ratio turns out to be about three times as great as the figure he used.
- There could be lots of normal, non-luminous matter present: This was perhaps the biggest objection to Zwicky’s conclusion. Why invoke some new type of matter to explain these galaxy’s motions within a cluster when the matter we know could be responsible? As long as it exists in any non-luminous form — gas, dust, black holes, low-luminosity stellar remnants, ionized plasmas, etc. — then it need not even be present in the individual galaxies, but could be found in the intracluster medium that separated these individual galaxies. With such a massive unknown, why leap to the extraordinary conclusion that some new type of matter not only exists, but dominates the Universe?
All of this, too, was before the development of multiwavelength astronomy: infrared, radio, and X-ray astronomy, that reveals so many of the normal matter components of the Universe that aren’t accessible in visible light. This was also long before the advent of gravitational lensing, which reveals the overall masses of massive cosmic structures like galaxy clusters.
As the evidence continued to pour in through the decades, it became clear that many of these common objections to Zwicky’s conclusions were, in fact, quite legitimate, and that Zwicky’s original “factor of 400” mismatch was quite far from the truth. Walter Baade’s work demonstrated that the Hubble constant Zwicky was using was far too large (drastically changing the distance estimate of these galaxies), and included an error which failed to recognize that the Cepheid variables he was using to measure galactic distances came in fundamentally two different types. This led to a Universe where the expansion rate was much lower, the distances were much closer, and the age of the Universe since the Big Bang was much older than Zwicky originally assumed.
As our understanding of stars improved, we realized that they accounted for significantly more mass than previously anticipated. Much of the light from stars doesn’t exist in the visible portion of the spectrum, but bleeds into the adjacent ultraviolet and infrared bands, revealing far more stars than visible light alone could. Then, starting in the 1960s, we began measuring X-rays from galaxies within galaxy clusters and — later — from the intracluster medium itself.
Clearly, Zwicky’s purported mismatch of “a factor of around 400 or more” between the observed amount of matter present and the gravitationally inferred amount of matter necessary to hold galaxy clusters together was not correct. Estimates of the ratio of those two values dropped from his initial value of around 400 to:
- 160, when we included the bias from the mismeasurement of the Hubble constant,
- to 50, when we improved our understanding of stars and starlight,
- to less than a factor of 10, when we began to understand just how much matter was present in forms like gas and plasma, particularly by studying the X-ray emissions of galaxy clusters.
Many continued to assume that all the hitherto undiscovered sources of normal matter would be able to eventually remove the need for “dark matter” almost everywhere. (The modern mismatch does indeed persist, but is only a factor of approximately 6. Clearly, 6 is closer to 1, which is the “no dark matter” scenario, than it was to Zwicky’s original factor of 400.)
However, if you considered all of the astronomical data available, there were still some hints of dark matter’s existence that stubbornly wouldn’t go away.
If starlight were a good tracer of matter — i.e., the matter is denser and more ubiquitous where the light from stars appears — then you’d expect the stars-and-gas in the interior regions of galaxies to zip around at greater speeds than the stars-and-gas on the outskirts. Galaxies, particularly spiral galaxies, are brightest and emit the most light from their central regions, with large bulges of stars concentrated in the center, and then the density of stars (and the brightness of the bulge and the disk) dropping as you move away from the central region.
The assumption we initially made was that there was no such thing as dark matter, and that mass appears where light also appears: a totally reasonable one. But as our astronomical capabilities improved, bolstered by the development of multi-wavelength astronomy (including X-ray, radio, and infrared astronomy) and the ability to achieve higher resolution in measuring both different parts of the same galaxy and smaller differences in velocity from place-to-place, the Universe started telling a different story than what we had assumed.
- Objects that were seen in the radio, first assumed and later confirmed to be galaxies, showed that the velocities of gas moving closest to the center were no larger than the velocities that could be measured farther away.
- More advanced measurements of galaxies in clusters showed a less-pronounced mismatch between the inferred mass from light and from gravitation than Zwicky first derived, but it still persisted.
- And from the balance between gravitational potential energy and the speed of stars in small structures — star clusters, globular clusters, and dwarf galaxies — it became clear that some sort of unseen mass was required to explain these smaller galaxies as well.
Even though people weren’t exactly synthesizing all of this together to make the case for dark matter’s existence, these independent lines of evidence certainly suggested that, perhaps, there was a seed of truth to Zwicky’s bold idea from all those decades before.
All of that sets the scene for the minefield that Vera Rubin walked into when she first began publishing her career-defining work on the rotation properties of individual galaxies throughout the 1970s. At this point in time, most astronomers were aware of Zwicky’s work, as well as the enormous sources of uncertainty surrounding the abundance of non-luminous matter that was still made from protons, neutrons, and electrons. The science of Big Bang nucleosynthesis was just coming into its own, and it was hinting that there was far too little normal matter to account for 100% of the energy that needed to be present in the Universe.
Meanwhile, a few galaxies exhibited rotation curves that were puzzling, and X-ray observations hinted at copious amounts of unseen but present “normal matter” within galaxy clusters. Importantly, the cosmological relationship between potential and kinetic energy in gravitationally bound objects — the virial theorem — had already become well-understood.
Working with her collaborator, Kent Ford, Rubin leveraged the new technology she had access to: Ford’s intensified image tube cameras. The spectra that she was able to take of different parts of the same galaxy were able to achieve high spectral resolutions and to image faint parts of the galaxy — portions that were farther away from the center — than ever before. Beginning with the Andromeda galaxy and extending her work to about ten other spiral galaxies, she saw what no one else had seen before: that all spiral galaxies exhibited relatively flat rotation curves, where the speed of the moving stars inside never dropped off to lower values, no matter how far away (within the observable limits) her measurements extended to. This is not what one would expect if stars, as traced out by the starlight, was what was responsible for the mass of galaxies. Instead, it should have behaved like the Solar System: where more distant planets orbit more slowly around the Sun than closer-in ones.
As groundbreaking as Rubin’s work was, it was equally controversial. Although the data was clear and unambiguous, the interpretation was not. The overwhelming majority of professionals in most sub-fields of astronomy were resistant to adding an entirely new form of matter to an already controversial Universe. Rubin was an observer, and the majority of referees — for non-scientific reasons ranging from inertia to clear-cut cases of sexism — demanded that she not include any interpretation of what the data meant in her published work. But Vera Rubin was brave, and not one to be intimidated so easily. She stood her ground, continued presenting her results, and left it up to the community to make what they would of it.
By the late 1970s, most astronomers began to be convinced by the strength of not only her data, but of the improving data in other areas — from X-rays, radio, Big Bang nucleosynthesis, and the rapidly developing field of large-scale structure — that all pointed toward the existence of dark matter. Over the next few years, many observations, including:
- of elliptical galaxies at various distances from the center,
- of individual stars within dwarf galaxies,
- of speeds of galaxies as they fell into larger-scale structures,
- and the quantitative discovery of (insufficiently large) amounts of normal matter in the intergalactic medium,
all helped astronomers reach the same conclusion. These multiple lines of evidence all indicated that the addition of a fundamentally new type of matter, what we today call dark matter, was required to explain all of these different observations together.
Today, the amount and quality of data at the disposal of all astronomers has improved by a factor of many thousands over what was available when Vera Rubin was engaging in her pioneering work. As is often the case, however, it’s unfair to credit a single person — even a single Nobel-worthy person who was snubbed as egregiously as Rubin was, or a brave pioneer with radical ideas and an inconvenient but strong piece of evidence like Fritz Zwicky — with the discovery of dark matter. Although both Rubin and Zwicky were a vitally important part of the story in bringing credibility and evidence that simply could not be ignored to the astronomical community, neither one of them “discovered” dark matter on their own.
Rubin, in particular, benefited greatly from the instruments available to her and from the previous work done in the field. Zwicky’s work in the 1930s, Horace Babcock’s early measurements of Andromeda’s rotation, Jean Einasto’s improvements to our understanding of the virial theorem and its applications to cosmology, Ivan King’s work on star clusters and dwarf galaxies, and Jim Peebles’s Nobel-winning work on the large-scale structure of the Universe all influenced not only her, but the greater astronomical community. It is only from synthesizing all that’s known together, from the full suite of evidence, that such a strong conclusion could be reached.
In truth, dark matter has no unique, singular discoverer, but rather only became accepted because of the full suite of astronomical evidence. As improved data came in over the subsequent decades, the case for dark matter became overwhelming, so much so that the only viable alternatives must also invoke an additional field whose properties are indistinguishable from the effects of dark matter. It wasn’t Zwicky nor Rubin who discovered dark matter, but it was both of them who paved the way for our modern, superior understanding of what it is that truly makes up the Universe.
This article was first published in July of 2023. It was updated in October of 2026.
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