Our “normal” matter is stuff made of atoms –like stars, planets, creatures –everything in nature. It’s easy to assume that everything in the universe should be made of atoms. But, in fact, only a tiny portion of the matter in the cosmos is the sort we are used to. Most –something like 84 percent of the matter in the universe is invisible and unidentifiable. Scientists call it “dark matter.” It does not interact with normal matter except (on very large scales) through gravity. To understand the nature and composition of our universe it is necessary to figure out what dark matter is.
Scientists determine the amount of matter in stars, galaxies and such by calculating the mass –a scientific form of weight. Recall that “weight” –like the weight of a person—changes in different conditions. You would “weigh” much more on a planet like Jupiter than on Earth, and you’d be “weightless” in space. But your “mass” would not change.
Astronomers routinely measure the mass of stars and galaxies. And long ago –back in the 1800s—some astronomers noticed something weird about their estimates. The mass of galaxies exceeded the calculations based on the visible matter in galaxies, and it was always a big difference.
In the 1920s, a Dutch astronomer, Jacobus Kapteyn, suggested the missing stuff might be a unique form of matter. In the 1930s, Fritz Zwiky, a famous astrophysicist at California Institute of Technology, went much further. Zwicky used one of the most advanced telescopes of that time to study the velocity of the galaxy clusters in order to calculate the mass of the clusters. He was astonished to discover that the actual mass holding those clusters together had to be hundreds of times greater than the figure derived from the observable light of galaxies. He called that invisible mass “dark matter.” And the name stuck.
Although Zwicky was famous and his calculations were clear, the scientific community basically side-stepped the issue of invisible mass for several more decades. Then, in the 1970s, a group of astronomers, including Vera Rubin, at the Carnegie Institute, Vera Rubin, calculated the rotation of spiral galaxies using a spectograph.
A fundamental law of physics dictates that the rotation at the edge of galaxies should be slower than rotation at the center. In fact, stars at the edge should sort of spin off. But Rubin and her colleagues found that the rotation speeds at the galaxy edge was equal to the speed at the center. This became “the galaxy rotation problem.” The only explanation for this condition was some mysterious force holding a galaxy together. The evidence of the Rubin group confirmed that galaxies contained disproportionate amounts of dark matter. And the confirmation changed the field of astrophysics and the general perspective about the universe.
Today scientists accept the fact of dark matter, but they cannot identify it. There are numerous ideas about what it might be: certain sub-atomic particles such as neutrinos and axions, for example. But there are no confirmations. Dark matter is certain, but certainly mysterious.