1933

Dark Matter

Fritz Zwicky (1898–1974)

We deal with unseen forces all the time—the wind blowing through our hair; gravity pulling us downward. We can make observations or run experiments that tell us these forces are there, however, and that can ultimately reveal their source. In 1933 the Swiss-American astronomer Fritz Zwicky encountered some new evidence for the action of unseen forces in the cosmos, but ran into a major, paradigm-changing roadblock because there was no apparent, or perhaps even measurable, explanation for what he saw.

Zwicky studied clusters of galaxies—some of the largest structures known in the universe. In one study, using spectroscopy, he was able to measure the red shifts and relative velocities of the members of the Coma cluster, a group of about one thousand co-moving galaxies about 320 million light-years from Earth. He discovered that the galaxies were moving relative to each other in a way that was inconsistent with their inferred masses. Even when Zwicky accounted for all the mass that he could see in visible light photographs, there still seemed to be an enormous amount—about four hundred times more than what he could see—of “missing mass” that was needed to explain the gravitational movements of the individual galaxies. This missing-mass problem led Zwicky to postulate that there must be some form of unseen matter, undetectable with then-modern methods, that was causing observed motions.

Even as new methods of radio, infrared, X-ray, and gamma-ray astronomy were developed, the missing mass in galaxy clusters—including apparently, within our own galaxy, based on the motions of nearby globular clusters—remained unseen. Astronomers now call this ubiquitous unseen material dark matter.

Many studies now require the existence of apparently undetectable material that has mass and exerts a gravitational influence on “normal” matter. Cosmologists believe it accounts for about 80 percent of all matter, making the mystery extremely profound and humbling. We appear to be a minor piece of a universe made of stuff that we simply don’t yet understand.

Images of the “bullet” galaxy cluster from the ground-based Magellan telescope and the Hubble Space Telescope (orange stars) and the Chandra X-ray observatory (pink gas) have been augmented with blue regions depicting the areas where computer calculations show that most of the cluster mass appears to be concentrated. However, the mass associated with the theoretical “blue” regions here remains unseen, or “dark.”