Section 1: Introduction
Figure 1: Distribution of dark matter in the universe.
Source: © Raul Angulo, Max Planck Institute for Astrophysics. More info
Dark matter is something beyond the stuff we encounter here on Earth. We all consist of neutrons, protons, and electrons, and our particle physics experiments with cosmic rays and accelerators tell us that a whole set of particles interact with each other to make up the world we see. As we learned in Units 1 and 2, the Standard Model describes these known particles and their interactions. But careful astronomical measurements, computer-based simulations, and nuclear theory calculations have all led us to believe that the particles described by the Standard Model account for only 4 percent of the mass of the universe. What makes up the missing 96 percent? Physicists believe, based on cosmological measurements described in this unit and Unit 11, that 23 percent is dark matter and 73 percent is dark energy. Dark energy and dark matter are very different. We shall learn about dark energy in Unit 11. Here, we focus on dark matter.
The first evidence of dark matter appeared in the 1930s, when astronomer Fritz Zwicky noticed that the motion of galaxies bound together by gravity was not consistent with the laws of gravity we learned about in Unit 3 unless there was a lot more matter in the galaxy cluster than he could see with his telescope. Development of more powerful and more precise theoretical and experimental tools in subsequent decades strengthened the case for dark matter. By the 1990s, dark matter was required to explain not just the motion of galaxies, but also how those galaxies and other large structures in the universe form, and the detailed pattern of temperature fluctuations in the cosmic microwave background radiation left over from the early universe.
Figure 2: The composition of the universe, with 96 percent invisible and unfamiliar.
With these distinct reasons to believe that dark matter is a real part of our universe, scientists struggled to understand what comprises dark matter. Could it consist of familiar objects like brown dwarfs and large planets—made of the stuff of the Standard Model, but not emitting light and therefore invisible to astronomers? Both theory and experiment eventually pointed away from this simple explanation, strongly suggesting that dark matter is something entirely new and different. A generation of experiments was developed to look for new types of particles—beyond the Standard Model—that could account for some or all of dark matter. In parallel, theorists have developed creative visions of what new physics could explain about the motion of galaxies, large scale structure, and variations in the cosmic microwave background in one fell swoop.
The process of discovery has not run smoothly. It has survived successive periods of disinterest, progressing as new technologies developed, scientists made fresh observations in disparate fields, and general scientific interest in the topic increased. In this unit, we describe why we think dark matter exists, its role in determining the structure of galaxies and clusters of galaxies, and how it connects with particle physics. Finally, we discuss the ongoing quest to determine what dark matter is made of in both theory and experiment.
Dark matter and gravity
The connection between dark matter and gravity bears special mention because it is the one thing about dark matter of which physicists are certain. Everything we know about dark matter so far comes from astronomy. The astronomical measurements deal exclusively with the way in which dark matter interacts gravitationally. We have two ways of studying the effects of gravity on astronomical bodies: We can either see how a group of astronomical objects moves under the influence of gravity or measure how gravitation changes the way in which light travels. Experimentally, we have no reason to believe that dark matter interacts with normal matter or with itself in any way other than via gravitation, although there is a great deal of theoretical speculation to the contrary.
Figure 3: Clusters of galaxies contain significantly more dark matter than visible matter.
Source: © ESO. More info
The effects of dark matter did not become apparent until astronomers began to study the motion of galaxies and clusters of galaxies. Since a galaxy that measures 150,000 light-years across contains 2 to 10 times as much dark matter as normal matter, the gravity from the dark matter plays a large role in its movements. However, the normal matter is clumped in solar systems (stars and planets), while the dark matter is spread out. Typical solar systems are about 10 light-hours across and are separated from each other by about 2 light-years. So, in conventional terms, the galaxy consists of mostly empty space interspersed with very dense clumps of normal matter.
Since a solar system contains far more normal matter than dark matter (2x1030 kilograms vs. 9x109 kilograms), dark matter plays an insignificant role in shaping our solar system. At the next level of size, observations indicate that normal and dark matter play roughly similar roles in determining the dynamics of galaxies. And at the largest-size scales, dark matter dominates the dynamics of galaxy clusters and superclusters—clusters of clusters. To study dark matter, we need to investigate objects the size of a galaxy or larger.