Dark Matter

I

Introduction

Dark Matter, in astronomy, designation for matter that does not give off or reflect detectable electromagnetic radiation, the radiant energy that includes visible light, radio waves, infrared radiation, X rays, and gamma rays. Although dark matter is practically invisible, astrophysicists have determined its existence by detecting its gravitational interaction with matter that does give off detectable electromagnetic radiation, such as stars, galaxies, and clusters of galaxies. Dark matter has become a vital component of modern theories of cosmology and elementary particle physics. Along with the phenomenon of dark energy, the puzzle of what dark matter is represents one of the most important questions in physics today.

II

The Discovery of Dark Matter

The existence of dark matter was first suggested in the early 20th century by the Swiss American astronomer Fritz Zwicky, but convincing and overwhelming evidence of its existence was gathered by the American astronomer Vera Rubin in the 1970s. In the early 1930s, Zwicky studied the rotational motions of thousands of galaxies clustered together in a large group of galaxies known as the Coma Cluster. He found that the orbital motion of the galaxies around their common center of mass could only be explained by the presence of unseen matter, which astronomers now call dark matter. Zwicky’s suggestion was not taken very seriously at first because there was not a great amount of evidence to support such a radical suggestion.

In the early 1970s, however, Rubin studied the orbital motions of stars in a large number of galaxies. As these stars orbited their galactic centers, Rubin noticed that the outlying stars in the galaxies were moving so fast that they should have been flung out of the galaxies. But since they were still part of the galaxies, Rubin proposed that unseen matter was keeping them gravitationally bound to the galaxies. A similar observation was reported in the early 1930s for the stars in our own Milky Way Galaxy by Dutch astronomer Jan Oort, but the dark matter interpretation was not considered at that time.

Following up on the observational data gathered by Oort, Zwicky, Rubin, and other astronomers, two American theoretical astrophysicists, Jeremiah P. Ostriker and P. J. E. Peebles, contributed important theoretical analyses. The data and the analyses helped scientists determine that dark matter probably constitutes as much as 90 percent of all the matter in the universe. Scientists verified that the orbital motions of stars in galaxies cannot be explained by the mutual gravitational influence of all the other visible stars. To explain this orbital motion, dark matter must be present. Rubin and other astronomers and astrophysicists showed that this dark matter seems to be distributed in a large envelope or “halo” around the visible matter of the galaxy. As a result the galaxies are much larger than what can actually be observed through a telescope.

---

---

Some scientists have theorized that there may be other explanations for the orbital motions of stars besides dark matter, such as a new type of force in nature that exerts itself over vast distances or a modification of the law of gravity. But so far, neither a new force nor a new understanding of gravity has been found. The hypothetical existence of dark matter, however, does explain the observed interactions with ordinary matter extremely well without resorting to a new long-range force.

III

What Is Dark Matter?

To gain a fuller understanding of our universe, it is vital to determine exactly what dark matter is made of. Scientists think that dark matter occurs in several different forms. Moreover, observations and experiments place limits on the quantity and distribution of each type. There are two broad categories of dark matter: “hot” dark matter, which moves at speeds comparable to the speed of light (about 299,000 km per second or 186,000 mi per second), and “cold” dark matter, which moves at speeds well below that of light.

A

Hot Dark Matter

The elementary particle called the neutrino, discovered in 1956, is an example of a hot dark matter candidate. Various experiments and observations, such as those reported in 1998 by the Super-Kamiokande experiment in Japan, have shown that the neutrino has mass. Mass is the quality that causes gravitational attraction. The mass of the neutrino is extremely small, which is why the particle travels at speeds comparable to that of light. Neutrinos are extremely abundant in the universe because they are produced in enormous numbers in nuclear interactions that take place at the core of every star. For example, several trillion neutrinos pass through each person on Earth each second as a result of the nuclear reactions that cause the Sun to shine. Because neutrinos are electrically neutral they can pass easily through ordinary matter, such as through people, and so are able to spread throughout a region near ordinary matter. Their large numbers could enable them to be a significant component of dark matter despite the tiny amount of mass in an individual neutrino.

However, there is evidence that dark matter cannot be made up mostly of neutrinos. This is due to two reasons. First, their likely mass is still too small to provide enough matter to account for the gravitational effects seen in the orbital motions of stars. In addition, some form of dark matter was necessary in the early universe to create the early structures that eventually led to the formation of stars and galaxies. Neutrinos could not have played this role, in part because they could not have been created in the required quantities until stars actually formed.

Secondly, neutrinos are too energetic to have helped seed the process of star and galaxy formation. Some other form of dark matter must have contributed to star and galaxy formation, which developed from localized structures, or lumps, known as anisotropies. These lumps have been detected in the cosmic microwave background radiation, the radiation left over from the formation of the universe in the big bang about 13.7 billion years ago.

Detailed observations of the cosmic background radiation show that these localized lumps in the early universe were too small to be seeded by fast-moving particles such as neutrinos. For the anisotropies to have formed in the early universe, a large component of slower, cold dark matter must have been present.

	More from Encarta
	•  Important Notice: MSN Encarta to be discontinued. Learn more. •  Presidential Myths Quiz •  Coffee break: Recharge your brain