One thing I’ve never understood about “dark matter”, and have never seen discussed in any article I’ve read on the subject, is why the stuff appears to segregate itself from normal matter. It appears to huddle on the outskirts of galaxies, which explains why we haven’t discovered the stuff until now (if it commingled, we would have noticed that everything was a lot heavier than it should have been, right?), but that doesn’t make a lot of sense to me. Can anyone explain?
Thanks,
Sua
I am not an expert but I understand that dark matter and regular matter anihilate each other when they come into contact. They don’t exist close together because anytime that happened, things would get destroyed so dark matter can only continue to exist in places where it is far away from regular matter.
Are you thinking of antimatter?
It sounds like you are thinking about antimatter instead of dark matter, Shagnasty.
First of all–and let’s be absolutely clear on this–nobody really knows what so-called “dark matter”, or more appropriately, “missing dark matter” (MDM), is. Technically speaking, most matter is “dark”, including yourself, your house, the planet you live on, et cetera; that is to say, it’s dark in the sense that it doesn’t radiate electromagnetic energy (other than what it absorbs from other sources, like the Sun) and can only be detected by gravitation or the afformentioned absoption and reradiation. The problem is that there is an enormous amount of mass–the majority of it, in fact–that appears to be missing in order for the mechanics of Newtonian and relativistic physics to work on large (galactic and larger) scales.
One set of proposals for this is that MDM is some kind of weakly interacting interstitial matter that flows not just around us but through us, exerting an immeasurably small local gravitational force but comprising such a vast bulk mass that its effects are apparent on large scale structures. There are, in fact, particles like this, called neutrinos which are leptons (elementry particles that don’t experience strong nuclear interactions) which lack a charge, so that they also don’t experience electromagnetic forces. The Sun vomits an enormous quantity of these as waste products from fusion such that trillions of them are pouring through you every second, but it takes a large and very sophisticated detector to see one even occasionally. However, missing dark matter is most likely not comprised of neutrinos, because while they fit the bill from a non-reactive point of view, the individual particles have a vanishingly small mass and the reactions that produce them send them flying at speeds which proclude them from gathering in structures as small as a mere galaxy, and also we detect no sign of the massive amount of neutrinos we should detect if it were the case that they were around and just moving a lot slower.
There’s a significantly more massive complementary family of supersymmetry particles called neutralinos which fit the bill much better; the problem with these is not only that we haven’t detected them but they actually are predicted by a hypothesis (supersymmetric extensions to the Standard Model of particle physics) which haven’t and can’t (with existing technology) be tested. The math works out pleasently enough, but the theory is a chimera, and doesn’t get us any closer to validating neutralinos or other exotic matter as comprising MDM.
It’s also possible that some of the MDM is ordinary matter that is just less visible than we think it ought be. It can’t be dust, because we should be able to detect thick clouds by their absoption, but at least some of it could be brown dwarfs, supermassive free planetary bodies, non-radiating singularities (black holes), et cetera, though it seems unlikely to astronomers and cosmologists that this could make up more than a small fraction of MDM. It could even be something more exotic like cosmic strings, or some form of concentrated gravitational energy we don’t know about. Or it could be that there is no MDM and that the gravitational force acts differently over large distances than we expect, or that General Relativity is incomplete, or that there are one or more additional forces that we can’t detect on everyday scales. (The latter is seriously unlikely, or would at least require a huge revision in our understanding of relativity and mass particle exchanges, but it’s not entirely out of the question.)
In any case, nobody thinks that MDM, if it exists, is “segregated” from regular matter; merely that we can’t see it or observe it in any way except for mass interactions, and that it’s gravitational effects on local scales are too small to be detected. If this seems suspiciously like the “luminiferous aether” explaination for the propogation of light from the 19th Century in terms of how vague and insubstantial it is, then perhaps you’re right; it’s a seriously incomplete hypothesis for a phenomena we still don’t really understand, and any physicst or cosmologist worth his diploma will admit such a thing up front.
Stranger
Wow. Nice guess, Stranger.
There was another dark matter thread less than two weeks ago.
Here’s another recent thread.
Collisions of matter on matter affect the distribution of matter in a galaxy. For example, if something is revolving “the wrong way” (i.e., its angular momentum about the core is perpendicular to the total angular momentum of the galaxy), it will repeatedly cross the rotation plane until it hits something which can send it into the plane. At steady state, things are distributed somewhat compactly / squashedly. (This mechanism works well for planetary rings since collisions out of the ring (and into the planet’s surface) will eventually rid the ring of misbehaving objects.)
If dark matter is weakly interacting, its distribution will not be heavily influenced by this collisional mechanism, so it can end up, as you say, “segregated”.
Oops, I sure was. I really do know the difference in real life 
This isn’t exactly my field of astronomy, but I’ll take a whack at this.
Dark matter doesn’t avoid the centers of galaxies, but it’s harder to observe it there with all the other stuff (normal matter, supermassive black holes, etc) going on in the center of a galaxy.
Most dark matter can’t be observed directly- technically, the Earth and all the stuff on it that doesn’t emit light is dark matter, and we observe things on Earth, so we can observe some dark matter. But even if all the dark matter were in the form of planets, which it isn’t, we can’t observe planets around stars in the galactic center or on the far side of the galaxy, let alone in another galaxy. All those extrasolar planets you might have heard about, with the exception of the pulsar planets, are within 200 light-years of the Sun. That’s very close compared to the size of the galaxy.
Since we can’t look for dark matter directly, we have to find other methods. One method, which is probably what you’re thinking of in the OP, is by observing galaxy rotation curves. Those show that the visible matter we see in galaxies can’t be all that’s there. They also show that the matter in the galaxy must extend out farther from the galactic center than what we see. The current theory is that there is a halo of dark matter (which here just means “something that has mass but we can’t see it”- galaxy rotation curves don’t tell us anything about the nature of dark matter except that it has mass) extending farther out from the center of the galaxy than the visible matter does.
Because of an electromagnetic phenomenon known as the Poynting-Robertson effect, dust grains in orbit tend to spiral inward. That dust helps in star formation. IANAGalaxy formation expert, but that’s my guess as to why the luminous part of the galaxy is so clustered around the galactic center. There’s evidence that 85-90% of the mass of the universe is made up of dark matter that doesn’t interact electromagnetically, so it wouldn’t be subject to Poynting-Robertson drag, and would be less likely to clump at the center of a galaxy.
I don’t know about you other guys but I sure radiate. That’s how night vision goggles work. Even at 2.7 deg A, stuff radiates. Sure it is in the microwave range, but it radates. And cold dark matter simply doesn’t. It doesn’t radiate and doesn’t absorb radiation either (two sides of the same coin). As far as is known it doesn’t respond to three of the four forces (electromagnetic, strong, and weak), but only to the ultrafeeble force of gravity.
I recently read something that cold dark matter doesn’t even respond to itself. There appears to be no clumping (except from gravity).
You can observe my mass when you pry it from my cold dar … ok, nevermind.
Whatever it is, it probably responds to the weak force, and might even respond to the strong force. That is to say, of the theoretical particles which have been proposed to explain dark matter, most of them are subject to the weak force (I can’t actually think of any examples which don’t, except perhaps microscopic black holes), and a few of them are subject to the strong force as well. But neither of the nuclear forces can have any significant effect over astronomical distances, as gravity and electromagnetism can.
Stranger on a train Thanks for getting this picture stuck in my head. “The Sun vomits an enormous quantity of these…” Now I’m always going to think of sunlight as solar-spew. I can’t wait to go outside and feel the sun barf on my face! 
It’s self-imposed segregation. Dark matter just feels more comfortable socializing with other dark matter.
Stranger,
This ezzack thing has been bugging my undereducated mind for years.
Is there any train of thought that gives not just a density concept to space, but instead of that density being a constant K, lets space vary in density, and lets space be smooth rather than particulate or quantized? (I’m using “space” in very much layman’s terms; whatever the substrate is within which we observe energy and particles and the like.)
In such a construct gravity would be a property of the relative density of space, rather than the more-often applied analogy of warping (with the natural path of a particle or light beam flowing down a density gradient…), all space contributes to the overall mass of the universe, dark matter clusters are areas where space is simply more dense, and Newton gets the last laugh arguing that without an aether there is no propagation of light. MDM is simply space itself, but with a quality closer to a paste of varying density rather that particles of any kind.
If dark matter exists in the form of particles that don’t interact with electromagnetism, then why haven’t they all collapsed into black holes? Electron repulsion is pretty much why there is any such thing as matter, instead of just a universe of gravity holes.
Well, here’s my problem; dark matter is supposed to be, what, 95% of the matter in the universe, but its gravitational effects on local scales are too small to be detected?! If 95% of the mass of the solar system was dark matter, the dark matter would mass more than the Sun. Wouldn’t that affect planetary orbits, or otherwise cause effects we would surely notice? And if dark matter isn’t in the solar system, why not?
Sua
Think of the earth. You would be weightless at the earth’s center because there would be equal amounts of mass in every direction pulling at you. The net effect is zero.
If the solar system were filled with dark matter in an equal mist everywhere, the effect would be similar. The pulls would cancel out and the orbits would be unaffected.
However, it’s not known whether dark matter actually fills all space, including between planets in the solar system, or whether it mostly forms a halo surrounding the galaxy. The latter is more likely and makes the question of dark matter’s behavior inside the solar system moot.
Black holes are unusual formations, created only when an extreme concentration of matter congregates. Most black holes we know of are at the center of galaxies, where the highest densities of matter exist.
Whatever dark matter is, it doesn’t congregate. It’s widely dispersed and possibly moving at a high speed. It has extremely low density. This makes it highly unlikely that it would collect at all, let alone to such an extreme extent that a black hole would form.