Zwicky's Dark Matter (1933): The First Hint of Invisible Mass

Fritz Zwicky discovered the first evidence for dark matter in 1933. Studying the Coma galaxy cluster, he found its galaxies moving so fast that the cluster would fly apart unless it held about 400 times more mass than its visible light suggested. He called the missing ingredient “dunkle Materie”: dark matter.

The question before the paper

In the early 1930s, galaxies had only recently been recognized as separate star systems, and astronomers were busy measuring their redshifts. Zwicky’s paper is mostly a review of that work. But it contains a pointed side question: in a big cluster of galaxies held together by its own gravity, do the speeds of the galaxies match the mass we can see?

What the paper found

• Fast-moving galaxies. From the velocities of a handful of galaxies, seven according to modern translators, Zwicky estimated that galaxies in the Coma cluster move around with a spread of roughly 1,000 km/s.
• Far too little visible mass. To hold galaxies that fast, the cluster’s average density would have to be about 400 times greater than the density derived from its luminous matter.
• A name for the gap. He described the missing component as “dunkle Materie”, dark matter.

How Zwicky discovered dark matter

Zwicky used the virial theorem, a rule from mechanics that links the average motion of objects in a stable, self-gravitating system to the total mass holding them together. Faster average motions mean more mass. He took the measured velocities of galaxies in Coma to estimate their spread, worked out the mass needed to keep the cluster bound, and compared it with an estimate of the mass in its stars based on their light. The difference was enormous.

Why it mattered

This paper is usually cited as the first evidence for dark matter, decades before the idea was taken seriously. Zwicky’s factor of 400 was inflated by the distance scale of the time, which was badly off, but the core conclusion has held: galaxy clusters are held together by far more mass than we see.

The idea won over most astronomers only around 1980. Vera Rubin and Kent Ford’s 1970 measurement of the rotation of the Andromeda galaxy found evidence of unseen mass in an individual galaxy, and Rubin’s survey of 21 spiral galaxies showed the effect is the rule. In 2006 the Bullet Cluster offered the most direct picture of dark matter separated from ordinary matter.

What we know now

With more than 1,000 velocities now measured in Coma, the cluster’s velocity dispersion turns out to be very close to Zwicky’s value. Today, based on the Planck satellite’s 2018 results, the universe’s content is about 68% dark energy, 27% dark matter and only 5% ordinary matter. What dark matter is made of remains unknown; the leading framework is cold dark matter, and experiments such as LZ search for its particles. See the dark matter guide and dark matter vs dark energy.

Who wrote it

Fritz Zwicky was a Swiss astronomer who worked most of his life at the California Institute of Technology, today home to Caltech astronomy and its Palomar Observatory. He published this paper in German, in the Swiss journal Helvetica Physica Acta.

Read the original

Zwicky, F. (1933). “Die Rotverschiebung von extragalaktischen Nebeln.” Helvetica Physica Acta 6, 110-127. doi:10.5169/seals-110267, free to read. An English translation, “The Redshift of Extragalactic Nebulae”, was republished in General Relativity and Gravitation in 2009 (doi:10.1007/s10714-008-0707-4), and English and Spanish translations are free on arXiv.

Questions people ask

Who first discovered dark matter?

Fritz Zwicky is usually credited with the first evidence, in 1933. He found that galaxies in the Coma cluster move too fast to be held together by visible matter alone and named the missing mass “dunkle Materie”. Galaxy rotation studies by Vera Rubin and colleagues in the 1970s and early 1980s made the case widely accepted.

When was dark matter discovered?

The first evidence came in 1933, from Fritz Zwicky’s study of the Coma galaxy cluster. Most astronomers were convinced only around 1980, when flat rotation curves of spiral galaxies showed unseen mass in individual galaxies. Its nature is still unknown as of October 2026.

Why did Zwicky overestimate the amount of dark matter?

Zwicky’s factor of about 400 depended on galaxy distances, which in 1933 were badly calibrated. Correcting the distance scale shrinks the discrepancy considerably. His measured velocity spread of about 1,000 km/s, however, is very close to the modern value from more than 1,000 galaxy velocities.