Astronomers are groping around in the darkness to find the darkness. There’s a whole universe out there, and they can’t really see any of it.

    There is more to the universe than meets the eye.

    Every crack is filled, every structure surrounded and permeated, and every seemingly empty reach of space crammed full by a unique and hitherto unguessed-at particle, trillions of which will pass through your body before you reach the end of this sentence. This particle is, in fact, an invisible and unbelievably small elephant which interacts with normal matter only through its gravity.

    Or, at least, it might as well be.

    As matters stand, you could make a case for that particle being practically anything, perhaps a full-grown elephant instead of a particle. While there is almost universal agreement now that dark matter exists in some form—and comprises quite a substantial portion of the universe—just what its form consists of remains a mystery. This puzzle has astronomers reexamining what they know and don’t know about physics and carefully pondering the history and future of the universe, from the earliest fraction of a second to its fiery—or perhaps cold and bleak—end.

    Although dark matter has only received significant attention during the past two decades, astronomer Jan Oort, namesake of the Oort comet cloud surrounding the solar system, first noted its existence during the 1930s. At the time, he was observing certain stars in the Milky Way which move up and down through the galactic plane in addition to displaying rotational motion. When he analyzed their motion, Oort found that the stars were behaving as though there was far more mass than anyone thought there was in the galaxy. In fact, he concluded, that there must be roughly twice the amount of visible mass in the Milky Way to account for the motion.

    Only a year later, something even more difficult to explain appeared. Fritz Zwicky, a Swiss astronomer working at Cal Tech, was studying the movement of galaxies in the Coma galactic cluster. Just as Oort had, Zwicky concluded that the objects he was watching were moving too quickly; the gravity of the observed mass of the cluster was not nearly enough to hold on to its members. Here, though, just as the scale of the structures was much higher than before, so too was the mass deficit. If Oort’s observation proved hard to explain, Zwicky’s was that much more difficult.

    Astronomers came to accept the existence of missing mass and forget about it. From these early observations to the 1970s, astronomers said little about the dark matter, a term Zwicky introduced.

    During the seventies, however, this dismissal became impossible. Vera Rubin—a Cornell alumna—performed a study of the rotation of spiral galaxies in an effort to understand the variation in the appearance of their arms. With a single measurement, she knocked down a long-held assumption about the movement of stars in a galaxy and brought the problem of the missing mass back to the foreground.

    Rubin, then working with W. Kent Ford, was recording the rotation curve of the Andromeda galaxy, the Milky Way’s next-door galactic neighbor. A scientist produces a rotation curve by measuring the relative speeds of the two arms of a spiral galaxy appearing edge-on from Earth. Since the galaxy spins, the light from one side of the galaxy will be Doppler-shifted towards the blue end of the visible spectrum, and the other side towards the red. Doppler shifting refers to the change in wavelength that results when a source of waves and the point of observation are in rapid motion with respect to one another. By comparing the two shifts, scientists can calculate the galaxy’s rotational speed.

    Rubin and Ford plotted the velocity as a function of the galaxy’s radius. When the two scientists examined the graph, they saw at once that it was all wrong: as the radius increased, the curve flattened out. Stars toward the outer reaches of Andromeda, which were supposed to orbit the galactic center more slowly than the interior ones, were instead moving at velocities similar to the stars closer to the center. They found the same results in other spirals.

    Dark matter had reared its head again. The only way the errant outer stars could be moving as fast as they seemed to be was if there was, in fact, far more matter keeping them bound to the galaxy than anyone thought there was.

    Since scientists made the first rotation curves, a wealth of new evidence has poured in, all suggesting that the universe is heaped full of dark matter. Around the same time Rubin and Ford were measuring red shifts, Princeton astrophysicists P. James Peebles and Jeremiah Ostriker put together a numerical simulation of a spiral galaxy. They found, to their consternation, that their model went haywire and disintegrated when they attempted to use observed matter distributions. Only when they embedded the visible galaxy in an outer sphere, or halo, of extra mass did they get a result which resembled the real thing. Since then, scientists have implicated extra matter in the rotation of globular clusters of stars around the center of our galaxy and in the gravitational lensing of very distant galaxies by nearer clusters of galaxies.

    Fine, so there is dark matter in the universe. Now what is it? Speaking broadly, it could either be composed of baryons—the ordinary matter that you and this magazine are made of—or something else. Just what that something else might be is not altogether clear.

    Part of the problem with defining just what dark matter is made of is that it’s very hard to say just how much of it exists. Estimates range from 10 or 20 times the amount of visible matter all the way up to 100 times. The true amount is critical, though, because it affects the possibilities at our disposal.

    Lower estimates of the missing mass—and thus the overall density of the universe—favor the baryonic suspects. These candidates include such structures as pockets of gas, black holes, brown dwarfs, and more mundane objects which are simply too faint to see, like white dwarfs.

    Black holes come in every conceivable size. Supermassive ones are thought to inhabit the centers of galaxies, while microscopic ones may randomly populate the heavens. Each type presents its own problems. Astronomers simply have not seen larger ones; tiny ones may evaporate over time and do not weigh very much. Brown dwarfs are objects too massive to be called planets, but which fall short of the critical mass needed to start thermonuclear fusion. White dwarfs are the retired lightweights of the stellar community. They represent the last stage in the evolution of a G-type star like our sun.

    The difficulty with many types of baryonic dark matter is that it is hard to see how they would remain separated from normal matter. If massive black holes, for example, populate the dark matter halos around galaxies, they should over time spiral inwards and collect at the centers. It is, however, possible to account for the mass deficit with everyday matter, as long as that deficit is no more than approximately 20 times what we can see. But if the collective dark matter sums to much more than this, we have a far greater problem.

    In the first blink of an eye after the Big Bang, all the matter which occupies the universe was created. From estimates of the quantity of matter created, some astronomers have placed an upper limit on the quantity of dark matter in existence today: roughly 20 times the amount of visible matter. Astronomers favoring a baryonic composition of dark matter point out that their calculations of dark plus visible mass come very close to the estimated abundance of mass created during the Big Bang.

    Some estimates of the missing mass, however, range much higher than this. If these calculations are correct, then the dark matter cannot be ordinary matter—at least not completely.

    There is a push in modern cosmology to bring the density of the universe to the “critical density”—the dividing line between a universe which will continue to expand outward forever and a universe which will eventually be overcome by gravity and collapse back in upon itself. The happy medium is a universe with a one-to-one ratio of actual to critical density, a quantity aptly referred to as W. The urgency to achieve this balance is largely poetic, perhaps, but there are sound physical arguments in favor of it. The difficulty of assigning this value to W is that it disagrees drastically with observations; measurements of the relative abundance of the light elements, which reflect the density of the early universe, indicate that we are nowhere near the critical density. In fact, visible matter comprises little more than 5 or 6 percent of the required mass.

    If we are at the critical density, then, the universe consists almost entirely of mass which remains largely invisible. And it is not even normal matter: far too much of it exists for it to consist of baryons alone.

    One of the simpler contenders is the neutrino, a tiny, chargeless fundamental particle. While they haven’t determined its mass—if it has any at all—scientists have, at least, established its existence, which is far more than one can say for some proposed candidates. If a neutrino possesses even a small mass, it could account for a majority of the missing matter; far more of them were created in the early universe than protons and neutrons. Evidence of massive neutrinos exists, but remains scant and highly debated. Yet if it should turn out that neutrinos are in fact massless, what else could the dark matter be?

    The possibilities prove endless. This is the point at which astronomers go for broke and particle physicists become cosmologists, attracted by the great particle hunt going on in the field next door. Collectively, these scientists classify their ideas as WIMPs—Weakly Interacting Massive Particles. (Appropriately, the baryonic candidates have been christened MACHOs—Massive Astronomical Compact Halo objects.) Scientists have not yet proven that WIMPs exist. WIMPs do, however, carry some attractive features.

    Supersymmetry theories—attempts to unify gravity, electromagnetism, the strong nuclear force, and the weak nuclear force—often rely on the existence WIMPs. Modern versions of the theories call for a number of relatively light particles which comprise the missing mass. These would be exotic creatures indeed, reacting only very occasionally with ordinary matter. One of the nicer aspects of these WIMPs is that calculations of their hypothetical quantity and distribution bring the density of the universe very close to the critical density.

    Bizarre as they seem, the WIMPs have the edge over the massive neutrino in at least one important matter: the formation of galaxies. Dark matter does not interact easily with light. At the time when galaxies were forming, light resisted the clumping of matter, tending to push apart particles before they had a chance to accumulate. Thus, dark matter proves necessary in the formation of galaxies: it could begin condensing to form galaxies before ordinary matter could.

    Massive neutrinos could not have done the job. With their tiny mass, they would move at speeds approaching that of light. Whizzing around in this manner, the neutrinos could not condense any more than baryonic matter could. The WIMPs, however, would carry more mass and consequently move much more slowly. A universe filled with cold dark matter could form galaxies on schedule as calculated, with accumulations of dark matter attracting the normal matter which, for us, comprises the galaxy.

    There are still other possibilities for dark matter, both MACHO and WIMP. There is the hypothetical axion, a particle deliberately named for a cleaning product. There is also the cosmological constant, Einstein’s infamous “greatest mistake,” which he introduced to counteract relativity’s predictions of an expanding universe, and which a few cosmologists think could effectively add energy, and thus mass, to empty space.

    With so many items on the menu, how are we to tell which is the real thing? Perhaps none of them are. Or perhaps the best solution is a mixture of all of them—a potpourri of WIMPs forming the galaxies, massive neutrinos flying around, and a selection of MACHOs just making things more interesting. Part of the uncertainty stems from the question of just how much dark matter there is. This, however, proves difficult to answer, depending as it does on the W factor of the universe, which depends upon the value of Hubble’s constant, which depends upon the measurements of distance and rotation of galaxies, which depends upon—and so on.

    It’s unlikely that the dark matter mystery will be definitely resolved anytime in the foreseeable future, and so this is a tale without an end, a problem without a solution. Perhaps we’re lucky to be living in an age of uncertainty about dark matter, for it is a time when we can be creative with our answers. We can dream about just what it is out there which wraps the galaxies in invisible shrouds and sits back, quietly laughing and waiting patiently to be named. We, like the dark matter itself, can afford to wait. It’s waited there these 15 billion years—or has it only been 10 billion? Or twice as much? Well, that depends on what value you calculate for Hubble’s constant, which—

    But that’s another story.

    Ignoring the cosmological implications of the argument, it seems easiest to believe in a wholly baryonic universe. With baryons, it’s possible to get some of the predictions matched up more or less neatly; exotic particles, while fun, should probably be held back as a last resort.

    Baryon fans can take some comfort from the fact that the known visible universe is getting heavier. Technological advancement has allowed astronomers to glimpse at incredibly faint and distant objects, including a whole host of galaxies and white dwarfs. In addition, better vision has revealed previously unseen features of the familiar universe. In particular, some galaxies appear to have higher concentrations of stellar and gaseous material than was hitherto thought to be the case. Most obvious, however, is the discovery of a handful of extrasolar planets or brown dwarfs, which has excited the public interest to no end.