Exotic matter and dark matterAlmost certainly most of the dark matter in the universe is non-baryonic and therefore made up of one or more forms of exotic matter. Here's why.
At the birth of the universe, in the Big Bang, some 13.7 billion years ago, all that existed was just an extremely hot soup of all sorts of particles. As the universe grew and cooled, the ordinary matter particles such as neutrons, protons, and electrons, started to join together to form atoms of the elements we see around the cosmos today – predominantly hydrogen and helium. Our theory of element-making in the first few minutes of cosmic genesis, called Big Bang nucleosynthesis (BBN), has been a great success. Not only does it predict that hydrogen and helium should be dominant but it gets them in the right proportions we see, allowing for changes that have happened inside stars since. But with this success comes a catch. The amount of each element that forms, it turns out, depends very sensitively on the amount of baryonic matter that the universe had available. Basically, BBN predicts the right ratios for the elements in the universe today only if the original amount of baryonic matter was no more than ten percent of the critical amount of matter needed to stop the cosmic expansion. Because scientists believe dark matter makes up far more than ten percent of the critical value, they're pretty confident that most dark matter isn't made of baryons. Of course, it could be that, despite having done such a good job so far, BBN will turn out to be flawed; some researchers are looking into that possibility. But the prevailing view is that whatever dark matter is, it isn't made of the same stuff we are.
So we have to look at the alternative. If dark matter isn't made of ordinary matter, which is baryonic, then it must consist of some kind of exotic matter, which is non-baryonic. In this sense, theorists use the term "exotic" as a catchall: it might mean something strange and new, but it doesn't have to. Some non-baryonic dark matter might consist of a particle that's been known about for many years: the neutrino. Wraithlike, billions of neutrinos from space (many from the sun) pass through your body, and then clean through the entire Earth, every second. Although aloof, they're one of the most populous particles in the universe and, in recent years, it's been found that, contrary to previous belief, they almost certainly have a small amount of mass. For a long time, physicists thought that neutrinos, like photons, traveled at the speed of light and therefore must have zero rest mass. But the discovery that neutrinos can change from one form to another, made within the last few years, implies that they can't be massless. Their mass, if confirmed, is certain to be tiny. But even if it's as small as one five-thousandth the mass of the electron (which, in turn is almost 2,000 times lighter than a proton) then, given the vast number of neutrinos cosmos-wide, this would still amount to enough dark matter to reverse the cosmic expansion. As it happens, however, there's a good reason, as we'll see in a moment, to suppose that neutrinos don't make up the lion's share of dark matter.
The other big non-baryonic dark matter hopeful goes by the unprepossessing acronym WIMPS. Weakly Interacting Massive Particles, to give them their full title, belong to a class of hypothetical heavy particles that hardly interact at all with familiar forms of matter (otherwise, they'd have been discovered by now.) You might think that since no one's ever seen an axion, a neutralino, or any of the other WIMP candidates, that there'd be a lot of skepticism surrounding this dark matter option. But, on the contrary, in the contest to be gravitational kings of the universe, WIMPS seem to be winning out over their biggest rivals, the MACHOS.
MACHO: Massive Compact Halo Object (the name chosen, in 1991, deliberately to contrast with WIMP). According to the MACHO point of view, sizeable galaxies like our own are cocooned by dark matter haloes that have a hefty population of non-luminous objects, including things such as brown dwarfs, other types of very dim stars, black holes, planets and so on. Although we wouldn't be able to see MACHOs directly, we could, astronomers realized, hope to detect them in another way. Any concentration of matter can, under the right circumstances, act like a lens, bending and focusing the light rays from a source that lies behind them at a much greater distance. Since the early 1990s, a number of projects have been on the lookout for "microlensing" events that would give away the presence of MACHOs. And they've had some success. In 2003, an extrasolar planet, a bit more massive than Jupiter, was found by this technique. But, overall, not enough MACHOs have been found to account for more than a fifth of all dark matter, and the final figure may be much less. Also, because MACHOs are presumed to be made of ordinary (baryonic) matter, they're contribution is likely capped by the BBN restriction mentioned earlier.
The case of the missing matter has yet to be solved but at least we've a good idea who the main suspects are: ordinary non-luminous objects (mostly in the form of MACHOs), neutrinos and WIMPS. And the strange thing is this. Although we know for certain that the first two suspects are real and make some kind of contribution to the dark matter total, its seeming more and more inescapable that the one that no one's ever detected – WIMPS – is the biggest component of all. Here's why.
We've seen that dark matter can be either baryonic or non-baryonic and that BBN (assuming it's correct) limits the amount of baryonic matter in the universe. That rules MACHOs out as a major dark matter component. Another way to categorize dark matter is into hot and cold varieties. Very light stuff that moves around at near light speed is called hot dark matter (HDM), and the top candidate for this is neutrinos. But we also know that, whatever dark matter is, it's the primary source of gravitation in the universe; so it must have played a crucial role in determining the structure of everything from galaxies to the large scale arrangement of superclusters (clusters of clusters) of galaxies. We know there's a lot of dark matter in galaxies because of the way that stars move in them. And we also know there's a lot of dark matter in galaxy clusters because of the way galaxies move in them. Hot dark matter wouldn't stay clumpy in this way. Nor would it have clumped together in the first place to play the important part it undoubtedly has in galaxy and galaxy cluster formation. These conclusions have been totally supported, in recent years, by results from the Wilkinson Microwave Anisotropy Probe (WMAP). It now seems beyond doubt that hot dark matter can make up only a small fraction of the dark matter total.
But when it comes to cold dark matter (CDM), the only viable candidate we've got that can account for the bulk of CDM is WIMPS. So, we're pretty much driven to the conclusion that WIMPS account for most of the dark matter in the universe. The only worrying thing, as we've mentioned, is that all this is based on inference – we don't have the slightest observational evidence that WIMPS exist!
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