Open Questions: Dark Matter

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See also: The standard model -- Supersymmetry -- CP symmetry violation -- Dark energy -- Magnetic monopoles

There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.

William Shakespeare - Hamlet, I.v.166


Types of matter


Observational evidence for dark matter

Baryonic dark matter

"Hot" dark matter: neutrinos

Supersymmetric particles


Magnetic monopoles, cosmic strings, membranes

Quark nuggets

Shadow matter


Dark energy and quintessence

Kaluza-Klein dark matter

Recommended references: Web sites

Recommended references: Magazine/journal articles

Recommended references: Books


Before we can talk about "dark matter", we need to begin by going over what is meant by "matter" in the first place. It seems intuitively obvious. Are things like rocks, water, air, planets, stars, and galaxies examples of matter? Yes. What about light, radio waves, X-rays? No, those are forms of energy.

But of course, the distinction isn't quite as clearcut as it may seem. According to the special theory of relativity, matter and energy are in some sense equivalent: E = mc2. Matter and energy can transform into each other. So what is special about "stuff" when it has the form of matter as opposed to energy? The answer, as far as we will be concerned, is that matter has a nonzero amount of "mass" -- the "m" in the equation above -- while energy does not. (Even this definition could be debated. Fundamental particles called "neutrinos" were considered to be matter long before it was established that neutrinos have nonzero mass. Fortunately, this is no longer an issue.)

Nevertheless, we've only shifted the question. Now the problem is to define what "mass" is. Before the general theory of relativity came along, physics recognized two manifestations of mass. First, there was mass as related to the force of gravity. Between any two material objects there is a gravitational force determined by the equation

F = Gm1m2 / r2
where G is a constant of proportionality (Newton's gravitational constant), m1 and m2 are the masses of the objects, and r is the distance between the centers of mass of the objects. From this equation it follows that if any object had zero mass, it would not experience a gravitational force with other objects. This is the main reason we count something as matter only if it has nonzero mass.

But there is another property an object can have, which is also called mass, and it enters into a different law. This is Newton's second law of motion, which states that the velocity of an object can change (in magnitude or direction) only if a force is applied. Acceleration is another name for change of velocity, and it is related to force by the equation F = ma, where F is the force, a is the acceleration, and m is the mass of the object.

Now, a priori, there is no reason to suppose that the property of an object called mass in this sense, more precisely inertial mass, because it measures inertia, i. e. resistance to change of velocity, is the same as mass in the previous sense, gravitational mass. However, no one ever observed a situation in which the two sorts of mass were different for the same object, so it was routinely assumed that they were the same, without enquiring too deeply about why this was.

In formulating the general theory of relativity, Einstein didn't explain this either, but he did make it a fundamental postulate, known as the equivalence principle, which says there is no way to distinguish the two sorts of mass. Stated differently, there is no way to distinguish the force on an object due to acceleration from the force due to gravitation, as long as all you can measure is the force itself, and not the motion of the object or the presence of a gravitational field. From this, it was possible to relate both types of mass to local curvature of space itself. Simply put, there is no mass anywhere space is locally flat, and nonzero mass only if space is curved to some degree at the location of the mass. Matter having mass "causes" space to curve, and the curvature produces the force known as gravity.

That's it. We really know nothing more about the nature of mass than that. If this is all we know, why bother even with this discussion? The answer is that it suggests two ways that astronomers can detect the presence of objects having mass -- matter, in other words -- and yet remain confident they are talking about the same thing. The first way is by looking at the motion of large visible objects like stars and galaxies. It must be possible to account for any observable motion in terms of the presence of matter, whether or not all the matter itself is visible. The existence of matter can be deduced from the motion of visible objects, even if the matter is not visible.

This was one of the great successes of Newton's theory of gravity and his laws of motion. It could predict very precisely how the planets of the solar system move, and could even account for changes in the orbits of asteroids and comets when they happened to come close to large planets like Jupiter. The theory predicted the existence of Pluto long before it was observed, because of peculiarities in the orbit of Neptune. One consequence of these laws of motion is that the planets do not revolve around the Sun as if they were attached to a rigid wheel. That is, each planet revolves around the Sun at a different rate, and the farther a planet is from the Sun, the longer it takes. In other words, the length of a year is different for each planet. It varies from about 88 days for Mercury to 248 years for Pluto.

Although the structure of a galaxy is rather different from that of a solar system, the same laws of orbital motion apply. A solar system with a single star is usually dominated by the mass of the star. In our solar system, the mass of the Sun is about 1000 times that of all the planets combined. In a galaxy there is usually no central object that is so dominant. Nevertheless, all the stars in the galaxy should revolve around the center of mass (also called the "center of gravity") of the galaxy more or less as if all of the mass were concentrated at the center. In particular, the farther away from the center of mass a star is, the longer it should take for the star to revolve around the center.

The shocking thing is, when this idea was actually put to the test, it was found to be false. Testing the idea wasn't all that easy. The problem is, with stars in our own galaxy, the Milky Way, it's not that easy to tell how far away most of them are, so we can't tell how far they are from the galactic center either. It's much easier to calculate distances of stars from the center in galaxies outside our own, simply because we can see the relative positions of the stars and the center, and so we know the distance of the star from the center as soon as we know the distance of the galaxy itself. Determining the velocity of stars is a little harder. It can be done by measuring the Doppler shift of a star's spectrum provided we are not viewing the galaxy from exactly above (or "below") the center. The only problem is that most stars outside our galaxy are so far away that recording their spectra was difficult with available telescopes until a few decades ago. In 1970 Vera Rubin finally made suitable measurements of the velocity of stars around the center of the Andromeda galaxy, our closest large neighbor galaxy, which is about 2 million light years away. She found that all stars had about the same velocity (around 150 miles per second).

It was as if the stars in Andromeda were attached to a rigid wheel. The most likely actual explanation is that there is far more matter associated with Andromeda than is visible as stars, and most of it is located outside the visible part of the galaxy. This was the first solid evidence for the existence of "dark" matter. Since 1970, a great deal of additional evidence has accumulated, as we will describe.

The notion that mass is associated with curvature of space also provides a second way it is possible to detect large amounts of matter even if the matter is dark and does not glow. Namely, it is possible to look for evidence of actual curvature in space. How is that done? Fairly simple, actually. One relies on the idea that light always travels in a "straight" line. However, in space which may be curved, "straight" has to be defined carefully. The actual definition of a straight line in curved space is a line which is the shortest distance between any two points that lie on the line. Distance, in turn, is measured in terms of a function called a metric, which defines the actual geometry of space. Mathematical properties of this metric make it possible to determine how much space is curved at any point, without having to go "outside" space to measure it. Einstein's equations of general relativity relate properties of this metric to the presence of mass.

But the immediately relevant point is that under exactly the right circumstances, we can detect curvature in space, and relate it to the presence of mass (and matter), by observing the path of light. This was how the general theory of relativity was first tested. In 1919 Arthur Eddington led an expedition to observe stars during an eclipse of the Sun. During the eclipse, stars could be seen very close to the edge of the Sun... and they were slightly displaced from where they "should" have been, because of the curvature of space caused by the Sun.

Using the same principle, we can now look for very distant objects such as quasars, and detect a large quantity of mass which lies in the immediate line of sight to these objects -- if there is any. Such an object may be a large galaxy or a galaxy cluster. If such an object is in the exact line of sight (which is a fairly rare occurrence), a phenomenon called gravitational lensing is observed, as the light from the distant object is magnified or distorted much as if it had passed through an optical lens. The amount by which the light appears to be bent is a measure of how much mass is in the intermediate object. In the case of a galaxy, for instance, there can appear to be 10 or 20 times as much matter present as can be accounted for by visible stars. This "invisible" matter is, again, called "dark" matter.

The question, of course, is: What does this dark matter consist of? We need to examine, systematically, the possibilities.

Types of matter

In the 19th century physicists and chemists verified the "atomic theory" of matter, which says that matter is composed of small units called atoms, and further, that there were just 92 types of naturally occurring atoms, corresponding to 92 naturally occurring chemical elements. By 1932 it was established that all atoms consist of just three types of smaller particles: protons, neutrons, and electrons. Still later, during the 1960s, protons and neutrons were found to be composed of yet smaller particles, known as quarks and gluons (with quarks accounting for most of the mass). Electrons still appear to be elementary particles, as do quarks, without further additional subparticles. We will call matter composed of protons, neutrons, and electrons (or equivalently, quarks and electrons) "ordinary matter".

All visible matter in the universe is composed of ordinary matter, as far as we can tell. Stars and the galaxies they make up account for the most obvious examples of visible matter, because they are luminous. Other objects such as planets, dust, and interstellar gas are also ordinary matter, but such matter may or may not be readily visible, since it in general is not self-luminous.

Planets in our own solar system are visible, of course, by reflected light, but planets in other solar systems, though known to exist, are not visible, since the light they reflect is too dim to be visible with current technology. This may change before too long, but only for relatively nearby solar systems. Additionally, in 2004 the first claim was made of the detection of an extrasolar planet by emitted infrared light. Dust around other stars can be detected, sometimes by the way it obscures the light from stars, sometimes by emitted infrared light.

Interstellar gas can be detected by light whose emission is stimulated by nearby hot stars, sort of like the glow of a neon light. Some interstellar gas is so hot that the light it emits is in the form of X-rays. Gas can also be observed by absorption lines caused in the spectra of stars that lie inside or behind the gas.

In addition, ordinary matter can occur in isolated clumps that are larger than planets but still unable to glow as a result of internally sustained thermonuclear reactions as occur in "real" stars. Some of these may be "failed" stars which are too small and cold to be capable of thermonuclear reactions and are known as brown dwarfs. Real stars that have simply burned up all their nuclear fuel and are now dark would also be in this category.

All particles of matter, such as protons and neutrons, which are composed of quarks are called baryons, from Greek barys, meaning "heavy". Electrons are not baryons. They belong to a category called leptons, from Greek leptos, meaning "small" or "light". An electron has only about 1/2000 of the mass of either a proton or a neutron (which have about the same mass). Neutrons have no electric charge, protons have an electric charge of +1, and electrons have a charge of -1. There seem to be a roughly equal number of protons and electrons in the universe, since there is no evidence for large objects with net electric charge or for a large number of free protons or electrons. Hence most of the mass accounted for by ordinary matter must be in the form of baryons. So this type of matter is also called baryonic.

The class of leptons includes several other types of particles. Two of these, the muon and the tau have properties very like those of electrons, except for a larger mass. A muon is about 207 times as heavy as an electron, and a tau is 3477 times as heavy as an electron. However, muons and taus are unstable, and they decay very rapidly, so they contribute essentially nothing to mass in the universe.

Aside from electrons, muons, and taus, there is just one other kind of lepton -- neutrinos. There are three types of neutrino, associated with electrons, muons, and taus. Only since 1998 has it been determined that neutrinos have nonzero mass. The mass of each type of neutrino has not yet been well measured, but it must be very small. There are other observational reasons for believing that the amount or mass in the universe due to neutrinos is fairly small, and this will be explained a little later. Although neutrinos are not baryons, and hence not part of baryonic matter, at least they are a kind of particle that has been observed and measured. It is estimated that matter in the form of neutrinos might has a mass that is about 10% that of baryonic matter.

High-energy particle physics today has a reasonably coherent theory that covers all types of particles which have been observed in matter to date. This theory is called the standard model of particle physics. It recognizes no other type of matter particles apart from baryons and leptons. (More accurately, there are also "force" particles such as photons and gluons. Photons are massless. Gluons are probably massless, but never observed in isolation. There are other force particles known as W and Z which have mass, but they are also not observed in isolation as they decay very quickly.) Therefore, if there is dark matter other than baryonic matter, it must be in some "exotic" form based on particles not described by the standard model.

In fact, there are two sorts of reasons for believing the universe must contain more matter in an exotic, non-baryonic form than there is ordinary matter -- quite a bit more. One sort of reason is theoretical, having to do with the way atomic nuclei were originally created very early in the Big Bang. This process is called nucleosynthesis, and it places a limit on how much baryonic matter can exist, given the relative proportions of several atomic nuclei observed to exist in the unverse at present. The other sort of reason is based on other kinds of observations. Some of these observations also limit the amount of baryonic matter that can exist, while others imply the total amount of dark matter is much above the limit of what can be baryonic, and hence there must be a great deal of matter which is not baryonic.

The nature of this non-baryonic dark matter is mostly not known at all, but we will talk about some of the possibilities. First, though, we'll look at nucleosynthesis and then the observational evidence for dark matter.


Protons and neutrons formed out of quarks approximately 10-5 seconds (i. e., 10 microseconds) after the Big Bang. The universe was actually quite a simple place at that time, as all the matter and energy was then in the form of protons, neutrons, electrons, neutrinos, and photons. (All of the anti-matter forms of these particles had already been annihilated.) Apart from these constituents, the universe could be pretty much completely described by its temperature and its density. The temperature was above 109 degrees Kelvin, 109 K, for short.

The density of baryons (protons and neutrons) is the key parameter of interest, because it is the number which tells us how much matter was in the form of baryons. The density at that time was vastly greater than it is now, because the universe has expanded so hugely since then. However, the number of baryons now is exactly the same as it was then. There are two reasons for this: (1) There were no anti-protons or anti-neutrons left, so they could no longer annihilate with protons and neutrons. (2) A neutron could change into a proton by emission of an electron (or vice versa if a proton captured an electron), but protons have a half-life of at least 1032 years and may in fact be completely stable.

The important thing is that we can estimate the density of baryons at that time from observations we can make today and the facts we know about the process of nucleosynthesis. The observation we can make today is that most of the matter that exists today in stars is in the form of just two nuclei: hydrogen (H) and helium-4 (He4). The proportions are about 75% H, 24% He4, and only 1% everything else. We know this from studying the spectra of stars, where the presence and amount of any chemical element can be determined from characteristic spectral emission lines. While there may be quite a lot of matter around that isn't part of stars, the proportion of different elements in that matter should not be much different from what is in stars, simply because all but the oldest stars have condensed out of the available supply of baryonic matter.

During the process of nucleosynthesis, two things are going on. First, the temperature is decreasing and, second, the density of baryons is decreasing. These are both consequences of the fact that the universe is rapidly expanding. Now, temperature is essentially the same thing as the average energy possessed by all particles. If the temperature and density are too high, nuclei consisting of more than a single particle cannot form, because if any did, they would be blasted apart by another collision. When the temperature and density are "just right" a proton and a neutron can collide and stick together to form a nucleus of deuterium (H2). Another collision with a neutron will produce helium-3 (He3). And finally, a collision with a proton will form He4.

Heavier elements can form, namely lithium and beryllium, but that is relatively improbable. H2 and He3 are energetically less stable than He4, so they tend to either build up to that nucleus or else decompose back into neutrons and protons. Further, when the temperature and density fall far enough, the probability of a collision to occur with enough energy to form a new nucleus becomes so small that the whole process essentially stops, and the proportions of the different nuclei do not change thereafter. The proportions established then are pretty close to what we still see today.

What we can compute is that in order to have the proportion measured today, the density of baryons at the time of nucleosynthesis must have had a certain value. There is a convenient way to express that density. The symbol Ω is used to stand for matter density. By convention, Ω=1 is the density of matter required for the universe to be perfectly flat in a global sense. Let Ωb be the actual density of baryonic matter. Then it can be computed from what is known about nucleosynthesis and the H:He4 ratio that the value of Ωb has to be about .05, or in other words, about 5% of the density required for the universe to be flat.

Observational evidence for dark matter

Recommended references: Web sites

Site indexes

Open Directory Project: Dark Matter
Categorized and annotated links. A version of this list is at Google, with entries sorted in "page rank" order.
The Net Advance of Physics: Dark Matter
An index of tutorial and research articles located at the physics preprint archive.
Dark Matter
A short list. More here.
Infography: Dark Matter -- Astrophysics
Lists a few printed references, as well as external links.
Galaxy: Dark Matter
Categorized site directory. Entries usually include descriptive annotations.

Sites with general resources

Explore the Science of Dark Matter
Part of the Cryogenic Dark Matter Search site. Contains some overview, a FAQ list, and a few external links.
Cryogenic Dark Matter Search (CDMS)
Project home page. Contains a description of the experiment, news stories, and a long list of links to other dark matter experiments.
UK Dark Matter Collaboration
A consortium of researchers conducting experiments to identify dark matter, especially involving heavy neutral particles. Good technical information. Has links to other dark matter searches.
Dark Matter and Gravitational Lensing
Home page of the Gravitational Lensing Group at the Institute for Astronomy, University of Edinburgh. The group uses gravitational lensing to study the distribution of dark matter.
Dark Matter Telescope
The Dark Matter Telescope is a proposed 8.4 meter, 7 square-degree field, synoptic survey telescope. The product of collecting area and field of view will be 20 times more powerful than any observatory now operating or under construction. It is also known as the Large-aperture Synoptic Survey Telescope (LSST).
The MACHO Project
A project searching for dark matter in the form of "massive compact halo objects".
The Mirror Matter Webpage
General information on the idea of "mirror matter", describing possible evidence, by Robert Foot.

Surveys, overviews, tutorials

Dark Matter
Article from Wikipedia.
Dark Matter: Introduction
Basic information on dark matter. Part of NASA's Imagine the Universe site. There's also a page on the nature of dark matter, and a more advanced level page on the topic.
Found: Most of the Universe
February 2007 article from SEED magazine, by Phil Plait. Explains how the COSMOS survey has mapped the distribution of dark matter in the universe.
Ask a High-Energy Astronomer: Dark Matter
Common questions, with answers, provided by NASA's Ask a High-Energy Astronomer service.
Dark Matter
Good, short overview, by Martin White. Contains some good external links. Also a page on hot dark matter.
Dark Matter
Excellent review article by Joseph Silk.
The Dark Side of the Universe
Slide presentation given by Joseph Silk at the 2001: A Spacetime Odyssey conference.
Dark Matter and Dark Energy
Slide presentation from September 2001 by Hitoshi Murayama. Provides a summary of observational evidence for dark matter and dark energy, as well as a quick overview of standard big bang cosmology.
Dark Matter Exposed: Animation Offers Clues to Cosmic Mystery
April 2003 article from Describes computer simulations of the movement of dark matter.
Dark Matter: Hidden Mass Confounds Science, Inspires Revolutionary Theories
January 2002 article from Discusses observational evidence for the existence of dark matter and reviews theories of what it could be.
Understanding Dark Matter and Light Energy
January 2001 article from Provides very brief summary of dark matter, and a few links that are more useful.
The Search for Dark Matter
A presentation by Roger Dixon, in PDF format, from an October 2001 conference on cosmology.
Dark Matter
A presentation by Evalyn Gates, in PDF format, from an October 2001 conference on cosmology.
Dark Matter
Lecture notes by Steve Lloyd, from a course on Elementary Particle Physics.
A Primer on Dark Matter
Part of Mike Guidry's Violence in the Cosmos site.
Dark Matter and Related FAQs
Answers to frequently asked questions. Some of the questions involve black holes, dark energy, and related topics.
Closing in on dark matter
June 2011 article from Physics World. Summary of interview with dark matter theorist Dan Hooper.
Discovering dark matter
June 2010 article from Physics World. "With the search for dark matter hotting up, Robert Crease proposes an experiment into the nature of discovery."
Prince of darkness
October 2009 article from Physics World. Interview with Alex Murphy, a senior investigator in the ZEPLIN III project searching for signs of dark matter.
A light in the dark?
August 2008 article from Physics World, by Edwin Cartlidge. "For the last 10 years physicists in Italy have been claiming to have directly detected dark matter, which is believed to make up 23 of the universe. The author finds out why their results continue to create controversy."
Shot in the dark
February 2007 article from Physics World, by Douglas Clowe and Dennis Zaritsky. "A cosmic collision between two galaxy clusters known collectively as the Bullet Cluster has provided researchers with persuasive evidence for the existence of dark matter."
Cleaning up dark matter
Summary of November 2006 article in Physics World, by Giovanni Bignami and Arnaud Dupays. "An experiment in Italy has found tantalizing but puzzling evidence for axions, one of the leading candidates for dark matter. The authors explain how a pair of spinning neutron stars should settle the issue once and for all."
Universe reveals its dark side
May 2005 article from Physics World, by Henrique Araujo. "Evidence for dark matter is growing, and so are our chances of directly detecting it."
Dark-matter dispute intensifies
April 2000 article from Physics World, by Frank T. Avignone. "Recent results from a dark-matter experiment in Italy suggest that the elusive weakly interacting massive particle or WIMP has finally been detected - but a rival experimental collaboration in the US disagrees."
The search for dark matter
January 2000 article from Physics World, by Nigel Smith. "Experiments housed deep underground are searching for new particles that could simultaneously solve one of the biggest mysteries in astrophysics and reveal what lies beyond the Standard Model of particle physics."
The Nature of Dark Matter
Excellent survey by Kim Griest. (Part of The Net Advance of Physics.)
The Dark Matter of the Universe
Part of the Cambridge Cosmology site. Has brief summary of evidence for existence of dark matter.
Dark Matter in the Universe
Useful page with explanations and external links, part of Gene Smith's Astronomy Tutorial.
Dark Matter Mystery
Very brief, simple description of dark matter as it relates to the Chandra X-Ray Observatory.
Dark Matter, Cosmology, and Large-Scale Structure of the Universe
A tutorial by Jonathan Dursi. Discusses evidence for dark matter.
Cosmic Hide and Seek: the Search for the Missing Mass
Good longer, but somewhat dated, 1995 overview article by Chris Miller.
Dark Matter
Single-page summary by Emad Iskander, Douglas Scott, Joe Silk and Martin White.
Where Is the Missing Matter?
Overview by Carlos Frenk.
Dark Matter
Overview of various ideas about dark matter from Stephen Hawking's Universe.
First sighting of dark matter
March 22, 2001 news article from PhysicsWeb about observations of white dwarfs that might contribute to dark matter.
Shadow cast on dark matter
October 2000 news article from PhysicsWeb about a variant interpretation of CMB measurements.
Dark matter claim meets resistance
February 2000 news article from Physics World, about a claim to have observed experimentally a weakly-interacting massive particle at the Gran Sasso laboratory.
Controversy reigns over 'dark matter' claim
August 21, 1998 news article from PhysicsWeb about purported observation of MACHOs.
A bizarre universe may be lurking in the shadows
March 2009 New Scientist article about experiments that are underway to search for dark matter in a variety of forms.
Does Dark Matter Encircle Earth?
Brief January 2009 Scientific American article, subtitled "Dark matter might exert measurable effects on Earth, the moon and gas giants."
Where's the Dark Matter?
September 2000 Scientific American news article about a theorist who believes that results from the Boomerang telescope provide evidence for "Modified Newtonian Dynamics".
What's the Matter?
May 2000 news item from Scientific American about problems with existing hypotheses about dark matter.
Galaxies shine light on dark matter:
May 2000 news article in Science News, about detection of gravitational lensing due to dark matter.
Mysterious Dark Matter
A brief overview at the Cosmos in a Computer site. See also More than Meets the Eye.
Is dark matter theory or fact?
June 1996 answer to a question in Scientfic American. Ask the Experts.

Recommended references: Magazine/journal articles

Mining for missing matter
Ron Cowen
Science News, August 28, 2010
In underground lairs, physicists look for the dark stuff.
Dark Riddles
Govert Schilling
Scientific American, October 2007
The Not-So-Dark Matter
George Musser
Scientific American, March 2007
Dark Influence
David Shiga
Science News, April 23, 2005
Scaled-Up Darkness
George Musser
Scientific American, September 2004
The Search for Dark Matter
David B. Cline
Scientific American, March 2003
Unveiling the Dark Universe
Ray Villard
Astronomy, November 2002, pp. 42-47
The Large-aperture Synoptic Survey Telescope (LSST), when constructed, will be capable of surveying 200 million objects per night. Although research into the distribution of dark matter is only one of many investigations to be enabled by the LSST, the information it provides about distribution should tell us far more than we now know about where dark matter is located and how it has affected the evolution of the universe.
Lurking in the Shadows
Adam Frank
Astronomy, January 2002, pp. 36-40
Shadow matter a type of matter conjectured to exist by certain versions of superstring theory. It is also unlike dark matter, yet could constitute a "parallel universe" similar to the one we know, yet apparently unable to interact with ours.
A Cosmic Crisis?
Ron Cowen
Science News, October 13, 2001, pp. 234-236
The cold dark matter hypothesis that does well in computer simulations of large-scale structure in the universe may have trouble with galaxy-size structure.
Through the Looking Glass
Ron Cowen
Science News, September 9, 2000, pp. 173-175
A hypothetical form of matter with analogues of all the known matter particles but able to interact with them only through the force of gravity may make up some or all of the universe's dark matter.
A Dark View of the Universe
Ron Cowen
Science News, January 8, 2000
Halos of invisible matter give galaxies surprising breadth.
On the Trail of Rogue Planets
Peter Catalano
Astronomy, December 1997, pp. 36-41
There are observations of gravitational lensing by a distant galaxy which suggest the existence of a huge number of "rogue planets", which are not part of any stellar system. Such planets could be a major component of dark matter.
What Is Dark Matter
James Trefil
Astronomy, July 1997, pp. 42-43
Very brief summary of the evidence for dark matter and some of the possibilities for what it could be.
The New Dark Age of Astronomy
Marcia Bartusiak
Astronomy, October 1996, pp. 36-39
Many types of evidence indicate that dark matter makes up the vast bulk of all matter in the universe. Since we don't know what it is, we don't know what the universe is (mostly) made of - but there are a variety of possibilities.
The Dark Side of the Galaxy
Ken Croswell
Astronomy, October 1996, pp. 40-45
One of the leading possibilities for the dark matter which occurs in galaxies like the Milky Way is known as MACHOs - massive compact halo objects. Observation of the phenomenon known as micro-lensing may help determine whether MACHOs rather than WIMPs (weakly-interacting massive particles) make up the dark matter, and even the sort of objects that might comprise the MACHO population.
Ghosts in the Cosmic Machine
Robert Naeye
Astronomy, October 1996, pp. 48-53
Dark matter on a cosmological scale may be neither MACHOs or very light particles such as neutrinos or (hypothetical) axions, but very massive non-baryonic particles called WIMPs.
Mapping Dark Matter with Gravitational Lenses
Anthony Tyson
Physics Today, June 1992, pp. 24-32
The amount of dark matter present in large galaxy clusters can be estimated from the gravitational lensing effect of the cluster.
The Dynamical Evidence for Dark Matter
Scott Tremaine
Physics Today, February 1992, pp. 28-36
Studies of the motion of luminous matter in galaxies and clusters of galaxies show that most of the mass must be in a form that is not visible. And the larger the scale that is considered, the greater the discrepancy tends to be.
Dark Matter in the Universe
Lawrence M. Krauss
Scientific American, December 1986, pp. 58-68
There is overwhelming evidence that less than 10% of the mass of galaxies consists of visible matter. The proportion of dark matter outside galaxies is probably even larger. There are a number of possibilities for what this dark matter may be.

Recommended references: Books

David B. Cline, ed. -- Sources and Detection of Dark Matter and Dark Energy in the Universe
Springer-Verlag, 2001
Proceedings of a symposium held in Marina del Rey in 2000. Although intended for specialists, most of it is understandable by anyone who has mastered a college astronomy course. This is a long volume with many papers, which reflects the breadth of the subject area -- the greatest part of the universe which is not directly observable (both dark matter and dark energy). Additionally, both theoretical and observational issues are covered. It's a good place to look and browse in order to get one's arms around many of the principle open questions in this mysterious topic.
Lawrence Krauss -- Quintessence: The Mystery of Missing Mass in the Universe
Basic Books, 2000
Although nominally about the subject of dark matter, this book uses that as a springboard to expound the main ideas of modern cosmology, and a great deal of particle physics as well. It's an outstanding piece of scientific exposition. The evidence is first presented that most of the universe must consist of some sort of "dark matter". Then each of the major possibilities is considered in detail. Perhaps the best aspect of the book is the emphasis placed on explaining how observational evidence supports what we already know and should -- in the future -- allow us to identify what really makes up the universe.
Michael Hawkins - Hunting Down the Universe: The Missing Mass, Primordial Black Holes, and Other Dark Matters
Perseus Books, 1997
Covers a lot of general cosmology in addition to evidence for dark matter. The author is a professional astronomer who advocates a theory that primordial black holes make up a major part of the dark matter.
Marcia Bartusiak - Through a Universe Darkly
Harper Collins, 1993
Popular survey of the history and current status of dark matter astronomy. Discusses possible forms of dark matter and means of detecting it.
D. W. Sciama - Modern Cosmology and the Dark Matter Problem
Cambridge University Press, 1993
Technical exposition of dark matter theory with a fair amount of mathematics by an eminent cosmologist.
Michael Riordan, David Schramm - The Shadows of Creation: Dark Matter and the Structure of the Universe
W. H. Freeman and Company, 1991
Good exposition of the physics of dark matter and its cosmological significance by a science writer and a theoretical physicist.
John Gribbin, Martin Rees - Cosmic Coincidences: Dark Matter, Mankind, and Anthropic Cosmology
Bantam Books, 1989
Overview of dark matter research by a science writer and a leading astrophysicist. Speculative material on "anthropic cosmology" is somewhat tangential.
Lawrence M. Krauss - The Search for Dark Matter in the Universe
Basic Books, 1989
In-depth survey of dark matter by a qualified theoretical physicist. Pretty good information on the physics of different possible types of dark matter.
James Trefil - The Dark Side of the Universe
Charles Scribner's Sons, 1988
Brief introduction to the observational evidence for dark matter and alternative possibilities for its nature.


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