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Physics for the 21st Century

Dark Matter

Most of the mass in galaxies like our own Milky Way does not reside in the stars and gas that can be directly observed with telescopes. Rather, around 90 percent of the mass in a typical galaxy is “dark matter,” a substance that has so far evaded direct detection. How can scientists make this astonishing claim? Because, although we have yet to detect dark matter, we can infer its existence from the gravitational pull it exerts on the luminous material we can see. Another facet of the dark matter problem comes at larger scales, where the total amount of mass in the universe exceeds the inventory of atoms we think were made in the Big Bang. A third indication of something missing comes from the evolution of the large-scale structure in the Universe, where fluctuations in the dark matter density are needed to seed the formation of the tendrils and filaments of galaxies we see in observations. So what is dark matter, and how might we find out? Determining the nature and distribution of dark matter is one of the most pressing (and most interesting!) open questions in modern science—it resides at the interface of particle physics, astrophysics, and gravity. Many candidates for dark matter have been suggested, from the ghostly axion (particles with a tiny amount of mass) to Weakly Interacting Massive Particles (WIMPs) that weigh in at 100 times the proton’s mass. In this unit, we shall review the observational and theoretical evidence for dark matter, and describe the attempts that are under way to find it.


Unit Glossary


The axion is a hypothetical particle that naturally arises in the solution to the strong-CP problem proposed by Peccei and Quinn in 1977. Axions are electrically neutral, and experiments have shown that their mass must be less than 1 eV. While they are relatively light particles, slow-moving axions could be produced in copious amounts in the early universe, and thus could be a significant component of the dark matter.

baryonic matter

In the context of cosmology, including discussions of dark matter and the evolution of the universe, the term “baryonic matter” refers to the so-called ordinary matter described by the Standard Model. The atoms and molecules we are made of are considered baryonic matter. Axions and WIMPs are examples of nonbaryonic matter. Most of the dark matter is presumed to be nonbaryonic; however, no nonbaryonic matter has been directly detected in experiments.

cosmic microwave background

The cosmic microwave background (CMB) radiation is electromagnetic radiation left over from when atoms first formed in the early universe, according to our standard model of cosmology. Prior to that time, photons and the fundamental building blocks of matter formed a hot, dense soup, constantly interacting with one another. As the universe expanded and cooled, protons and neutrons formed atomic nuclei, which then combined with electrons to form neutral atoms. At this point, the photons effectively stopped interacting with them. These photons, which have stretched as the universe expanded, form the CMB. First observed by Penzias and Wilson in 1965, the CMB remains the focus of increasingly precise observations intended to provide insight into the composition and evolution of the universe.

CP violation

The CP operation is a combination of charge conjugation (C) and parity (P). In most interactions, CP is conserved, which means that the interaction proceeds exactly the same way if the CP operation is performed on the interacting particles. If CP is conserved, particles with opposite charge and parity will interact in the same way as the original particles. CP violation occurs when an interaction proceeds differently when the CP operation is performed—particles with opposite charge and parity interact differently than the original particles. CP violation was first observed in neutral kaon systems.

dark forces

Dark forces arise in a 2009 theory to explain various experimental results in high-energy astrophysics. The theory proposes that dark matter WIMPs can decay into force-carrying particles, denoted by the Greek letter phi (phi.gif). The phi.gif particles would be associated with a new force of nature, distinct from the strong force, weak force, electromagnetism, and gravity.

dark matter

Dark matter is a form of matter unlike the ordinary matter that is described by the Standard Model. It accounts for most of the mass in the universe, but only has been observed indirectly through its gravitational influence on ordinary matter. Dark matter is believed to account for 23 percent of the total energy in the universe.

Doppler shift (Doppler effect)

The Doppler shift is a shift in the wavelength of light or sound that depends on the relative motion of the source and the observer. A familiar example of a Doppler shift is the apparent change in pitch of an ambulance siren as it passes a stationary observer. When the ambulance is moving toward the observer, the observer hears a higher pitch because the wavelength of the sound waves is shortened. As the ambulance moves away from the observer, the wavelength is lengthened and the observer hears a lower pitch. Likewise, the wavelength of light emitted by an object moving toward an observer is shortened, and the observer will see a shift to blue. If the light-emitting object is moving away from the observer, the light will have a longer wavelength and the observer will see a shift to red. By observing this shift to red or blue, astronomers can determine the velocity of distant stars and galaxies relative to the Earth. Atoms moving relative to a laser also experience a Doppler shift, which must be taken into account in atomic physics experiments that make use of laser cooling and trapping.

equivalence principle

The equivalence principle is a basic premise that is essential to every experimentally verified physical theory, including General Relativity and the Standard Model. It states that an object’s inertial mass is equivalent to its gravitational mass. The inertial mass of an object appears in Newton’s second law: the force applied to the object is equal to its mass times its acceleration. The gravitational mass of an object is the gravitational equivalent of electric charge: the physical property of an object that causes it to interact with other objects through the gravitational force. There is no a priori reason to assume that these two types of “mass” are the same, but experiments have verified that the equivalence principle holds to a part in 1013.

galaxy cluster

A galaxy cluster is a group of galaxies bound together by the force of gravity. Like the planets in our solar system, galaxies in a cluster orbit a common center of mass. However, galaxies execute more complicated orbits than the planets because there is no massive central body in the cluster playing the role of the Sun in our solar system. Galaxy clusters typically contain a few hundred galaxies, and are several megaparsecs (ten million light-years) in size. The orbital velocities of galaxies in clusters provide strong evidence for dark matter.

gravitational lensing

Gravitational lensing occurs when light travels past a very massive object. According to Einstein’s theory of general relativity, mass shapes spacetime and space is curved by massive objects. Light traveling past a massive object follows a “straight” path in the curved space, and is deflected as if it had passed through a lens. Strong gravitational lensing can cause stars to appear as rings as their light travels in a curved path past a massive object along the line of sight. We observe microlensing when an object such as a MACHO moves between the Earth and a star. The gravitational lens associated with the MACHO focuses the star’ light, so we observe the star grow brighter then dimmer as the MACHO moves across our line of sight to the star.


The gravitino is the superpartner of the graviton. See: superpartner, supersymmetry.


The graviton is the postulated force carrier of the gravitational force in quantum theories of gravity that are analogous to the Standard Model. Gravitons have never been detected, nor is there a viable theory of quantum gravity, so gravitons are not on the same experimental or theoretical footing as the other force carrier particles.


An ion is an atom with nonzero electrical charge. A neutral atom becomes an ion when one or more electrons are removed, or if one or more extra electrons become bound to the atom’s nucleus.

ionization electron

An ionization electron is a free electron moving at high speed that knocks an electron off a neutral atom, turning the atom into an ion.


A light-year is the distance that light, which moves at a constant speed, travels in one year. One light-year is equivalent to 9.46 x 1015 meters, or 5,878 billion miles.


A MACHO, or massive compact halo object, is a localized mass that has a gravitational influence on the matter around it but does not emit any light. Black holes and brown dwarf stars are examples of MACHOs. MACHOs were once thought to make a significant contribution to dark matter; however, gravitational lensing surveys have demonstrated that most of the dark matter must be something else.


The mini-computer was a precursor to the personal computers that are ubiquitous today. Prior to the development of the mini-computer, scientists doing computer-intensive calculations shared mainframe computers that were expensive multi-user facilities the size of small houses. Mini-computers cost ten times less than mainframe computers, fit into a single room, and had sufficient computing power to solve numerical problems in physics and astronomy when fully dedicated to that purpose. When mini-computers first became available, many areas of scientific research blossomed, including the study of how structure formed in the universe.


MOND, or Modified Newtonian Dynamics, is a theory that attempts to explain the evidence for dark matter as a modification to Newtonian gravity. There are many versions of the theory, all based on the premise that Newton’s laws are slightly different at very small accelerations. A ball dropped above the surface of the Earth would not deviate noticeably from the path predicted by Newtonian physics, but the stars at the very edges of our galaxy would clearly demonstrate modified dynamics if MOND were correct.

N-body simulation

An N-body simulation is a computer simulation that involves a large number of particles interacting according to basic physical laws. N-body simulations are used to study how the structures in our universe may have evolved. Typically, many millions of particles are configured in an initial density distribution and allowed to interact according to the laws of gravity. The computer calculates how the particles will move under the influence of gravity in a small time step, and uses the resulting distribution of particles as the starting point for a new calculation. By calculating many time steps, the simulation can track the growth of structures in the model system. Depending on the initial density distribution and cosmological parameters selected, different structures appear at different stages of evolution. N-body simulations have provided strong support to the idea that our universe consists primarily of dark energy and dark matter. These simulations are resource intensive because the number of interactions the computer must calculate at each time step is proportional to the number of particles squared. A sophisticated N-body simulation can require tens of thousands of supercomputer hours.


The neutralino is the superpartner of the neutrino. See: neutrino, superpartner, supersymmetry.


Neutrinos are fundamental particles in the lepton family of the Standard Model. Each generation of the lepton family includes a neutrino (see Unit 1, Fig. 18). Neutrinos are electrically neutral and nearly massless. When neutrinos are classified according to their lepton family generation, the three different types of neutrinos (electron, muon, and tau) are referred to as “neutrino flavors.” While neutrinos are created as a well-defined flavor, the three different flavors mix together as the neutrinos travel through space, a phenomenon referred to as “flavor oscillation.” Determining the exact neutrino masses and oscillation parameters is still an active area of research.


The term “nucleosynthesis” refers either to the process of forming atomic nuclei from pre-existing protons and neutrons or to the process of adding nucleons to an existing atomic nucleus to form a heavier element. Nucleosynthesis occurs naturally inside stars and when stars explode as supernovae. In our standard model of cosmology, the first atomic nuclei formed minutes after the Big Bang, in the process termed “Big Bang nucleosynthesis.”


Parity is an operation that turns a particle or system of particles into its mirror image, reversing their direction of travel and physical positions.


Singularity is a mathematical term that refers to a point at which a mathematical object is undefined, either because it is infinite or degenerate. A simple example is the function 1/x. This function has a singularity at x = 0 because the fraction 1/0 is undefined. Another example is the center of a black hole, which has infinite density. In our standard model of cosmology, the universe we live in began as a spacetime singularity with infinite temperature and density.

Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS) is one of the most extensive and ambitious astronomical surveys undertaken by modern astronomers. In its first two stages, lasting from 2000 to 2008, SDSS mapped almost 30 percent of the northern sky using a dedicated 2.5 meter telescope at the Apache Point Observatory in New Mexico. The survey used a 120-megapixel camera to image over 350 million objects, and collected the spectra of hundreds of thousands of galaxies, quasars, and stars. Notable SDSS discoveries include some of the oldest known quasars and stars moving fast enough to escape from our galaxy. SDSS data has also been used to map the distribution of dark matter around galaxies through observations of weak gravitational lensing and to study the evolution of structure in the universe through observations of how both galaxies and quasars are distributed at different redshifts. The third phase of the survey is scheduled to end in 2014, and is expected to yield many exciting scientific discoveries.


A superconducting quantum interference device, or SQUID, is a tool used in laboratories to measure extremely small magnetic fields. It consists of two half-circles of a superconducting material separated by a small gap. The quantum mechanical properties of the superconductor make this arrangement exquisitely sensitive to tiny changes in the local magnetic field. A typical SQUID is sensitive to magnetic fields hundreds of trillions of times weaker than that of a simple refrigerator magnet.

strong-CP problem

The strong-CP problem is a particular inconsistency between experimental observations of strong interactions and the theory that describes them. Unlike in weak interactions, CP violation has never been observed in strong interactions. However, the theory that describes strong interactions allows CP violation to occur. So why is it not observed? That is the strong-CP problem. Various attempts to resolve the strong-CP problem have been proposed, but none have been experimentally verified. See: axion, charge, CP violation, parity.


In the theory of supersymmetry, every Standard Model particle has a corresponding “sparticle” partner with a spin that differs by 1/2. Superpartner is the general term for these partner particles. The superpartner of a boson is always a fermion, and the superpartner of a fermion is always a boson. The superpartners have the same mass, charge, and other internal properties as their Standard Model counterparts. See: supersymmetry.


Supersymmetry, or SUSY, is a proposed extension to the Standard Model that arose in the context of the search for a viable theory of quantum gravity. SUSY requires that every particle have a corresponding superpartner with a spin that differs by 1/2. While no superpartner particles have yet been detected, SUSY is favored by many theorists because it is required by string theory and addresses other outstanding problems in physics. For example, the lightest superpartner particle could comprise a significant portion of the dark matter.


The WIMP, or weakly interacting massive particle, is a candidate for what may comprise the dark matter. WIMPs interact with other forms of matter through gravity and the weak nuclear force, but not through the electromagnetic force or the strong nuclear force. The lack of electromagnetic interactions means that WIMPs are nonluminous and therefore dark. They are assumed to be much heavier than the known Standard Model particles, with a mass greater than a few GeV. WIMP is a general term that can be applied to any particle fitting the above criteria. The neutralino, the supersymmetric partner of the neutrino, is an example of a WIMP candidate.

Content Developer: Peter Fisher

Peter Fisher is a professor of physics and the division head of Particle and Nuclear Experimental Physics at MIT. His main activities are the experimental detection of dark matter using a new kind of detector with directional sensitivity and understanding the weak interactions using tau decays detector with the BaBar detector. His other projects include neutrino physics, wireless power transfer, pedagogical work on electromagnetic radiation, and development of new kinds of particle detectors.

Fisher earned a B.S. in Engineering Physics from the University of California, Berkeley, in 1983, and a Ph.D. in Physics from the California Institute of Technology in 1988.

Featured Scientist: Doug Finkbeiner

Doug Finkbeiner is assistant professor of astronomy at Harvard University. As a postdoctoral student at Princeton, he studied the Galactic microwave emission in the Wilkinson Microwave Anisotropy Probe (WMAP) data, finding signs of the long-suspected spinning dust emission. Another curious (and controversial) result from WMAP is the unexpected excess in the inner Milky Way, the “haze,” which may be synchrotron emission from cosmic-ray electrons and positrons produced by dark matter annihilation. This possibility is now being investigated at the Harvard-Smithsonian Center for Astrophysics by Finkbeiner and his group. They have recently written a paper entitled “A Theory of Dark Matter,” tying together many observation results in high-energy astrophysics with a unified model of dark matter.


Featured Scientist: Rick Gaitskell

Rick Gaitskell is a professor of physics and head of the particle astrophysics group, at Brown University. He and his group are focused on directly detecting the rare interactions between dark matter in the form of WIMPs (weakly interacting massive particles) and ordinary matter by employing a novel detector technology based on a target of xenon. He has been hunting particle dark matter for over twenty years. His latest experiment is the LUX (Large Underground Xenon) experiment, a large two-phase liquid/gas xenon dark matter detector and water shield which is being constructed deep underground at the Sanford Lab located at the Homestake Mine in South Dakota.


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Physics for the 21st Century


Harvard Smithsonian Center for Astrophysics in association with the Harvard University Center for the Environment. 2007.
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  • ISBN: 1-57680-883-1