The puzzle began in 1933, when Fritz Zwicky noted that galaxies in clusters -- linked together by gravity -- orbit around each other far faster than would be justified by the amount of mass visible in their glowing stars, and that therefore most of their mass must be "dark" in some form.

The problem was intensified in 1975, when the Carnegie Institution's Vera Rubin used a new, sensitive Doppler spectrograph to determine the speed at which the stars within galaxies themselves orbited around the galaxy's center. She discovered, to her astonishment, that -- in the case of almost all galaxies -- starting at a certain limited distance from the galaxy's center (usually very limited), the stars orbited around it at virtually the same speed no matter how far they were from the center.

You don't have to be a physicist to realize that, according to Isaac Newton, this doesn't add up if the galaxy's mass was indeed concentrated at its center in the way that its distribution of visible stars indicated. There apparently had to be a great deal of something invisible spread out through the galaxy -- and for a considerable distance beyond its visible edge -- that had mass and was itself revolving around the center of the galaxy, with the gravitational pull from its spread-out mass keeping the visible stars turning around the galaxy's center at a steady rate.

Galaxies had long been known to possess a roughly spherical "halo", with a relatively sparse sprinkling of visible stars, gas and dust, beyond their central main bodies. But the mass that such visible halo materials must possess didn't even begin to approach the mass necessary to explain the velocity behavior of galaxies' stars.

There had to be something else. And the more astronomers looked at galaxies, the bigger that amount of "something else" had to be compared to the stuff we can see with our various types of telescopes. Our own Milky Way has about 90% of its mass in the form of dark matter; the figure for the average galaxy seems to be about 95%, in the form of a spherical halo that extends several times beyond the distance of the visible halo.

The evidence for the stuff then continued to mount rapidly. When we were able to put X-ray and infrared telescopes on satellites above Earth's atmosphere, their unprecedented detailed maps of the distribution of hot gas and dust between the individual galaxies in clusters meshed perfectly with Zwicky's observations of visible galaxies early in the century.

There had to be a lot of something invisible spread out through such clusters -- with enough mass not only to keep the fast-circling galaxies from flying apart from each other, but to keep the very rarified but extremely hot gas (whose atoms were at temperatures of millions of degrees and thus moving very fast) from spreading out and dispersing away from the cluster.

Even calculations of the density of matter that must be necessary right after the Big Bang -- in order to make the smoothly spread-out plasma and gas of the initial Universe start clumping together gravitationally at anything like a fast enough rate to form the galaxies and individual stars that we see in today's Universe (let alone in the young universe) -- indicated that there had to be a vast amount of this invisible "something" back then, whose mass was exerting enough additional gravitational pull to make those initially rarified clouds of gas start pulling themselves more closely together, despite the very intense blast of high-energy photons throughout the early universe that was trying to heat and disperse them again.

And when it finally became practical to start detecting "gravitational lensing" -- that remarkable process by which the gravitational pull of a mass of murder bends the trajectory of light rays around it -- the results backed up the dark-matter theory still more.

This includes both the measurements we can occasionally make of a galaxy's or cluster's mass if it's positioned just right for its gravitaional lensing to actually produce symmetrical duplicate images of another galaxy in line beyond it. And it also includes the weaker lensing effects that we can detect now that our telescopes can see a huge number of extremely distant galaxies, so that we can statistically map the patterns by which the images of those background galaxies are slightly distorted by gravitational lensing fairly near the line of sight to a nearer galaxy or cluster.

All these different pieces of evidence add up: the Universe as a whole seems to contain fully almost 6 times more dark matter than it contains of stars, gas and dust that we can see using any telescopic technique. (By the way, even in the case of the matter we can see, the mass of the Universe's spread-out gas is fully 9 times that of the amount of it that's clumped so far into stars.)

But there were still a few holdouts, who based their opposition mostly on the simple fact that we haven't been able yet to determine what the devil the mystery stuff might be. The theorized candidates for it fell into two general groups, which astronomers (with their sometimes twee penchant for cutesy acronyms) christened "MACHOs" (MAssive Compact Halo Objects) and "WIMPS" (Weakly Interacting Massive Particles).

The former would be astronomical-sized but relatively small objects that we simply can't see because they're dark, such as very small and dim red dwarf stars, small white dwarfs produced when such small stars finally run out of nuclear fuel, smaller "brown dwarfs" (objects between 13 and 60 times the mass of Jupiter, which are massive enough to carry out nuclear fusion of the traces of deuterium and lithium that initially existed in them, but not dense enough to fuse their regular hydrogen as stars do), still tinier "rogue planets" floating separately in interstellar space, or even large numbers of black holes produced by burned-out stars.

Really big, galactic-mass sized black holes can be ruled out in any adequate number by the fact that our gravitational-lensing maps haven't detected them; and even relatively cool interstellar and intergalatic gas and dust can now be seen and measured by us.

But there are now three very strong arguments that MACHOs can't possibly make up more than a small fraction of dark matter. The first is that we have a very good understanding of the efficiency with which atomic nuclei of deuterium must have first fused together out of collisions in the dense, fast-moving sea of separate protons and neutrons that existed during the first few minutes after the Big Bang, and then themselves collided further to form the supply of helium in the universe -- and the ratios of these elements and isotopes in the Universe today point very strongly toward the density of "baryons" (protons and neutrons; that is, ordinary matter) in that young post-Big Bang universe being much smaller than its total density today of visible-plus-dark matter.

The second is that the subtle spatial patchiness of the "cosmic microwave background" -- the leftover glow from the Big Bang itself -- is pretty clearly a result of the patchiness in the distribution of such primitive gas in the very early Universe -- in fact, that patchiness reflects actual gigantic sound or density waves in the gas, whose wavelength depends on the density of the gas. Our calculations of the density of the Universe's regular baryonic gas this way fits very well with our calculations from the ratios of light elements noted above -- and, again, it's much lower than the Universe's total density of visible-plus-dark matter.

Finally, our ability to make detailed light-lensing maps has now reached the point that we can use the technique for "microlensing". This involves actually doing a survey of the number of small objects in our own galaxy's halo (and in the halo of the giant nearby galaxy in Andromeda), by seeing how often their gravitational fields happen to make the light of more distant stars in the small "Magellanic Cloud" galaxies near our own galaxy -- or in the Andromeda galaxy -- twinkle as the MACHOs cross briefly in front of those more distant stars. (In fact, our microlensing abilities are now so good that we're also starting to use the technique to try and count the average number of planets orbiting the stars in the Milky Way.)

These microlensing surveys, run in the 1990s, are consistently indicating that only about 20-30% of the Milky Way's dark matter can consist of such objects. This, yet again, fits pretty well with those other two pieces of evidence. And it also rules out the possibility that there might be a huge number of small black holes that didn't start out as stars and thus as chemical elements at all, but were instead directly created by the conditions in the Universe right after the Big Bang. (The MACHOs that do exist, by the way, tend to run about half as massive as our own Sun, which indicates that most of them are either small red dwarf stars, or small white dwarfs created from such small stars after they run out of nuclear fuel.)

All this indicates that most of the dark matter in the Universe is not any kind of "normal" baryonic matter made out of atoms or out of the protons and neutrons that make up those atoms. It would seem that it has to be some different kind of matter. But what?

One possibility would be that it consists of neutrinos -- those tiny, ghostly particles given off by some types of radioactive decay, and also produced in utterly gigantic numbers by the Big Bang itself. Neutrinos, once created, are famous for being incredibly reluctant to interact again with regular matter -- the average neutrino could pass through several light-years of lead before finally interacting and being stopped by any atomic nucleus in it -- and they also don't respond to the electromagnetic force, which means that they neither give off light nor affect its passage through them in any way.

That is, they really are at least one proven form of "dark matter". In the 1990s, it was also finally confirmed that (contrary to previous theory) they aren't devoid of mass. Each neutrino actually does possess an extremely small mass -- as little as one five-millionth to 1/250,000th the mass of an electron -- but their incredible numbers mean that they must make up a significant fraction of the universe's mass.

But neutrinos are so incredibly lightweight -- and traveled at such high speeds during the universe's early days -- that they whizzed around the Universe in an evenly spread-out way. Their light mass and high speed kept them from clumping together tightly enough under the influence of their own gravity, or of the gravity of visible matter, to come anywhere near serving as the means by which the gas of the early universe could be quickly clumped together to form galaxies and galactic cluster early on.

They could have been slowed down enough by their mutual attraction to set off vastly more spread-out and rarified clouds, which could possibly have been the formative power behind "superclusters" -- those gigantic clusters made of multiple galactic clusters themselves, which are hundreds of millions of light-years across as against "only" tens of millions of light-years for clusters. But we now have very strong evidence on several fronts that the gas of the early universe started to clump together into smaller galaxies and clusters considerably before those smaller clouds were later drawn together to form superclusters -- and so the dark matter which enabled that early smaller-scale clumping couldn't have consisted of neutrinos.

So the current leading opinion is that most dark matter consists of some kind of much heavier, slower-moving subatomic particles that therefore do respond to the tugging of gravity to clump together, concentrating themselves around visible galaxies and overall clusters of galaxies. Thus their acronym "WIMP". There are a whole variety of heavy particles predicted by different theories of subatomic physics that might fill the bill -- plus the "axion", a theorized particle that is even lighter than the neutrino but travels at far slower speeds and so does get drawn in and clumped together more easily by gravity fields.

Precisely because Weakly Interacting Particles do interact so weakly with particles of ordinary matter and with photons of light, it will be very hard to detect them and confirm their existence, let alone discover what their precise characteristics are. (It took 23 years from the theoretical prediction of neutrinos to the first successful detection of them.) But, whatever their precise nature, at this point they would appear to be by far the most likely candidates for dark matter.

At this point, however, that small group of remaining dissenter physicists comes in. They posit that the phenomena we've observed that seem to point toward the existence of dark matter -- the high speeds at which galaxies in cluster orbit each other; the similar high speeds with which the stars in the outer parts of galaxies orbit around the galaxy's center; the apparent need for a lot of extra mass-possessing but dark matter to make visible-matter stars and galaxies grow at all in the early universe; the gravitational lensing effects that seem to indicate the presence of a lot of otherwise-invisible mass-possessing material clumped around galaxies -- could be explained instead by a revised theory of the behavior of gravity itself: "MOND" (MOdified Newtonian Dynamics).

In this theory, when a gravitational field is below a certain low strength level, its effect on matter suddenly changes drastically -- it actually becomes a lot more efficient in pulling at matter than such a weak field would be expected to be. Thus we don't see this difference in a local gravitational field as strong as the Sun's is for us, or even in the inner part of the galaxy where the collective gravitational field of all the stars is still fairly strong -- but in the outer fringes of the galaxy, where the overall gravitational pull of the galaxy is less on its outer stars, it actually has a much more dramatic effect on their behavior than one would expect, making them orbit around the galaxy's center at a higher speed than such distant stars would normally do. Thus the effect of each galaxy's gravitational field "spreads out" and dilutes with distance much less than it would otherwise do.

The initial version of the MOND theory, developed in 1983, was rather impromptu -- its creator didn't even make any attempt to integrate it with Einstein's findings. But a revised, much more detailed version -- TeVeS ("TEnsor-VEctor-Scalar theory"), developed only in 2004 -- claims to successfully integrate the MOND idea with General Relativity's concept of gravity as actual curved space, by adding two more terms to Einstein's equations. It claims to adequately explain the gravitational lensing effects on light itself that have been attributed to large amounts of dark matter, and also to lead to the accumulation of enough locally concentrated clouds of matter in the young post-Big Bang universe to explain both those gigantic sound-wave patterns seen in the Cosmic Microwave Background glow and the fact that galaxies and stars were able to start clumping together quickly in the young universe. All of this without any dark matter at all.

So, what now? The latest piece of very dramatic new evidence in the Dark Matter mystery was revealed last September, with new observations by team led by the University of Arizona's Douglas Clowe which they claimed constituted (to quote the title of their article) "a direct empirical proof of the existence of dark matter".

It involved observations of the distant "Bullet Cluster", a spectacular example of two galactic clusters colliding at very high speed. This happens fairly often, as do actual collisions between individual galaxies. But in this case it happened only 100 million years ago, making this by far our best view of what things look like immediately after such a collision of clusters.

The two galactic clusters hit at fully 17 million kilometers per hour. As always in such cases, the stars comprising all the galaxies in the two clusters slid past each other virtually without noticing. Stars are so tiny compared to the immense distances between them that, if our own galaxy collides with the similarly huge Andromeda Galaxy 3 billion years from now -- as it may or may not do, depending on whether the two approaching galaxies are moving sideways compared to each other -- the odds are over a thousand to one against even a single pair of their combined 1.4 trillion stars colliding!

But the interstellar gas between stars, and between galaxies in a single cluster, is a very different matter. While it's incredibly rarified, since it fills up those same vast interstellar and intergalactic distances its total mass (as I said earlier) is actually far more than the total mass of all the actual stars in the galaxies in this case, about ten times more. And when two such vast clouds of gas fly through each other, their individual atoms -- even given the large spaces between them -- collide over and over, greatly heating the gas clouds (in this case, by a million degrees C.) so that they give off X-rays. And that same collisional friction drastically slows down the two gas clouds -- the stars comprising the two galaxies or two clusters continue flying on their merry way, but all the gas between them is stripped away from them by the collision, with the two gas clouds emerging from the collision site at much slower speed and getting left behind by the stellar parts of the galaxies. In the case of the Bullet Cluster, the two groups of stars from the two galactic clusters are now about 2 1/3 million light years from each other -- but their slowed-down, left-behind gas clouds are only about half that far apart.

What did Clowe's team do? They used "weak lensing" -- that statistical analysis of the slight distortions in the shapes of the images of hundreds of more distant galaxies behind the Cluster and vastly more distant from us -- to determine just how powerful the gravitational fields from the different types of matter associated with this collision were, and where that matter was located. What they found was two massive central concentrations of gravitational force from the matter in the two clusters, enough to indicate about seven times more mass of matter than the mass of the gas in the two visible glowing gas clouds. And the centers of those concentrations were unquestionably not the gas clouds; they were the concentrations of stars in the two clusters, still flying onwards on their merry way -- with those stars containing only about 1/70th the mass of the invisible dark matter that the lensing had detected wrapped around them. (By the way, the detailed observations of the two superheated gas clouds were made by NASA's big X-ray detecting "Chandra" orbiting telescope, while the Hubble Telescope was used along with two big ground-based telescopes to make the gravity-lensing map. So two of NASA's four orbiting "Great Observatories" were both crucial to making this major discovery.)

Modified theories of gravity predict that the gravity from galaxies or galactic clusters fades much more slowly with great distance than Newton or Einstein thought -- and that this can "counterfeit" the lensing effect that one would get from a far greater concentration of mass if those traditional gravitational fields were correct, thus making it appear that there's a huge additional supply of invisible matter wrapped around the cluster when there really isn't. But had that been the case this time, that huge lensing would have been centered squarely around the two slowed-down gas clouds, since they still contain about 10 times more total mass than the stars of the two clusters. It's not. It's centered instead around the two faster-moving clouds of stars instead, indicating that most of the mass in each galactic cluster copied the behavior of the stars, and so must consist of stuff that can fly right through another cloud of itself without being slowed down in the slightest, like a ghost. That is, dark matter.

This observation by itself doesn't give us any additional evidence as to whether that dark matter consists of MACHOs (which would have raced past each other just like the stars), or WIMPs -- clouds of subatomic particles that don't interact with each other electromagnetically. Since those electromagnetic forces, linking atoms together and linking the electrons in each atom to its nucleus, are entirely responsible for what we laughingly call the "solidity" of matter, such WIMP clouds pass through each other easily, affected only by their gravitational attraction, which first accelerates them faster toward each other and then, after the collision, slows them down again by the same amount. But dark matter, whatever it consists of, has had its existence proven beyond doubt at last.

Or has it? It takes a long time for any story in astronomy to be conclusively resolved -- and, sure enough, there are still a few holdouts on this seemingly ironclad proof of "collisionless dark matter". A team of American, Chinese and Belgian astronomers, led by Hong-Shen Zhao at Britain's University of St. Andrews, published a piece this January claiming that the Bullet Cluster certainly proves the existence of dark matter -- but that, if you utilize the TeVes modified gravity theory, the only dark matter required to explain it is a relatively small amount of mass in the form of regular neutrinos, whose existence has been long known -- rather than a larger amount of some as-yet unknown type of matter. Specifically, they claim that only about three times the mass of the visible glowing gas is required in the form of neutrinos to explain the Bullet Cluster's lensing -- an amount which may very well exist in those galactic clusters, if you assume that neutrinos have about 1/250,000 the mass of electrons (which is a very real possibility).

Given the limits we can place right now on neutrinos' possible mass, and the number of them that must have been created by near-certain post-Big Bang conditions, they can only weigh somewhere between one-tenth and 3 times as much as the gas in the clusters. But the TeVeS theory would, once again, predict that those two relatively lightweight clouds of neutrinos -- having passed ghostlike through each other, and so still evenly pacing the two retreating clouds of visible stars that also passed smoothly through each other during the collision -- would exert gravitational effects at long distances much more powerful than those predicted by Newton and Einstein, once again generating an unexpectedly powerful light-lensing effect that would "counterfeit" the effect of a much larger mass of some other unknown kind of collisionless dark matter if Newton and Einstein are correct.

Canadian physicist J.W. Moffat has proposed a wilder alternative idea: that the correct theory to explain gravity may be one called "MOG" (Modified Gravity), in which gravity has an unexpectedly powerful effect not at weak field-strength levels (as with MOND and TeVeS), but at great distances -- and that, in the case of the Bullet Cluster, the maximum lensing effect of the gravity from the two clouds of glowing gas may happen by pure luck to be at just the right distances from the two gas clouds to roughly line up with the location of the two more distant clouds of visible stars. In his theory, we don't need any significant dark matter at all, whether large amounts of ordinary neutrinos or anything else to explain the Bullet -- just the gravity from the two glowing gas clouds themselves. But he admits that so far he's worked this out mathematically only in one-dimensional form -- that is, his theory could explain why the maximum lensing is at a certain distance from the gas clouds, but it doesn't yet explain why that lensing takes the form of two spatially limited clouds at that distance, rather than the form of a couple of big doughnut-shaped rings around the two gas clouds at that distance from them. Combine this with its apparent need of a pretty lucky coincidence, and it's hard for this writer to see how Moffat's theory can be made plausible. But Zhao's "combined TeVeS-neutrino" theory may be more plausible.

More recently, a team led by Johns Hopkins' Myungkook Jee has found a still stranger piece of visible evidence for dark matter through their analysis of photos taken by the sensitive "Advanced Camera for Surveys" on the Hubble Telescope, of of the aftermath of another galactic cluster -- this one fully 5 billion light-years from us. While the Bullet Clusters' two colliding clusters were moving almost at a right angle to our line of sight when they hit, giving us a direct "side-on" view of the collision's aftermath, in this new case Jee claims that the ACS photos show the lensing effect from a great circular "smoke ring" of invisible dark matter, 2.6 million light-years across, that has been blown straight toward us by another such collision. He thinks that the galactic cluster is moving toward us (relatively speaking, if one ignores the expansion of the universe as a whole), and that 2 billion years ago it hit another cluster which was racing straight away from us -- dispersing the approaching cluster's dark matter into such a spreading ring because of the gravitational tug between it and the retreating cluster's dark-matter cloud as they passed each other during the collision. We can't directly see the second, retreating cloud of stars because it's behind the nearer one, and thus obscured from our view. (By this view, the two clouds of dark matter we see in the case of the Bullet Cluster -- if we could see them at a 90-degree angle to our actual side-on view of them -- would also be revealed as expanding "smoke rings" of dark matter.)

In this case, however, the lensing produced by the possible ring is much fainter and harder to see -- indeed, it may yet turn out to be a chance artifact of the distribution of light sources seen in the ACS' photos. There may thus never have been a collision at all in this case. And Hubble's ACS, unfortunately, broke down shortly after taking these images. Although it was supposed to be the third most important of Hubble's five instruments after the planned final 2008 Shuttle expedition to renovate Hubble, NASA is still trying to decide whether that expedition will try to fix it -- there's a lot on the repair mission's plate as it is.

Finally, even if the "smoke ring" is real, if it does exist -- since it's circularly symmetrical -- it not only would not provide us with any more evidence to distinguish between Clowe's "WIMP" dark-matter theory and Zhao's "TeVeS-plus-neutrino" theory; it can't even provide any evidence against Moffat's theory that the lensing effect that it shows was entirely produced by the quirks of long-range gravity, with no dark matter at all.

So science's biggest ghost hunt, alas, still continues -- not only do we still not know what dark matter is; we may still not be certain that it exists at all, or that it's made of anything but the neutrinos that we already know exist. And on top of that, we now have the even more bizarre and mysterious phenomenon of "dark energy", discovered wholly unexpectedly in the late 1990s -- some kind of apparent energy, stored in the structure of space itself, which is making the universe actually expand at a steadily faster rate, and which is so powerful that if it were converted Einstein-style into an equivalent amount of mass, it would weigh three times more than all the matter (dark and otherwise) in the universe. Cosmology is still crammed with almost as many fundamental still-unanswered questions as astrobiology is; the two will be competing with each other for a long time to come as the most "glamorous" branches of space science.

Bruce Moomaw is our first "Space Blogger" at www.spaceblogger.com Feel free to create an account on SpaceBlogger and discuss this issue and more with Bruce and friends.

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