So, you know that we cannot "see" dark matter, but we can detect it. But I bet your wondering about the quantity, distribution, and location of the "unseen" material. Well, isn't that a coincidence, we are about to discuss such aspects of dark matter.

Please Note: This page is fairly long, but stick with us--we do have a point!

A Further Note: We very strongly suggest watching the videos available at this page, as they tend to retard the drowsy effects of boredom.

Before we are able to discuss the distribution and location of dark matter, we must first examine the quantity (or amount) of this "lightless" matter. In reality, the quantity of dark matter greatly affects the distribution and hence, the location of the "invisible" stuff. The ultimate fate of our universe has perplexed scientists for ages partly because it hinges on one unknown parameter: the total mass of the universe. If the universe has an overall high-density then it is considered "closed." If the universe is indeed "closed," then mutual gravitational influence will eventually stop the universe's expansion and cause it to contract--which may result in a big crunch, followed by perhaps re-expansion. On the other hand, we may live in a low-density, or "open" universe. If we do live in such an "open" universe, the current expansion will eternally continue. There is also a model for a universe with "critical" density--one that is delicately balanced somewhere between high and low density. In this model, the total universal mass is just right and will cause the universe to coast to a gentle stop, where it will remain (no longer expanding, yet perpetually resistant to contraction). As far as we can tell, the universe is probably "open," and thus, destined to grow forever. We can calculate this by adding up the total mass of the stuff we can see, plus all the dark matter inferred from high planetary and galactic velocities. Comparing this total to various universal calculations, there exists only a fraction--about 20 percent--of the mass required to cease the universe's expansion. But who is to attack the possibility that entire "dark" systems, or even galaxies might lay beyond the edge of our known universe. At the present time, we simply don't know; we have no data/evidence to confirm or deny the existence of such unseen systems.

Okay, we now know that the absolute quantity of dark matter in the universe is unknown. However, of the stuff we can see and calculate, there is only 20% of the mass needed to halt the expansion of the universe--as far as we know. And of that, as much as 90% of the matter in existence may be dark matter. So I'm willing to bet that your wondering how all this "invisible" stuff is distributed. Well, there are basically three sides (with variations of each) to this controversy. The dark matter material may be in heavily concentrated areas, dispersed all over the place, or some percentage of both. The first argument basically states that all of the dark matter is concentrated in a few areas in each galaxy/system. When the theory of dark matter first came out, some astronomers and physicists believed that a gigantic central mass could explain the high velocities of galaxies. They said that a humungous ball of extraordinarily dense stuff was at the center of every galaxy. And, individual systems within galaxies rotated around this huge entity. It was so massive that it must be composed of some particle(s) that are unknown and unobserved to us. They believed a super-massive black hole (a gravitationally completely collapsed object with such a strong gravitational pull that not even light can escape) at the centers of each high-speed galaxy could be responsible for such a gravitational tug. When one examines this theory on dark matter, the flaw becomes obvious: the effects of such a massive object could theoretically be seen from Earth. Moreover, The influence of dark matter is not affecting single galaxies alone. In fact, it appears some galaxies are orbiting (or on a collision course to) others. A mass at the center would therefore be impossible because if such a gravitationally strong object did exist (as in it could influence other galaxies) most of the stuff in the galaxy itself would already be sucked in--which is disproved simply by our existence. The second argument of distribution states that all the dark matter is scattered about. We would like you to know that this is hard to explain without revealing how it is possible (as that is discussed on the next page). Suffice to say, the second argument says that there must be some type of particle that is all around us (other than those carrying light) which somehow has a minute, almost non-existent, mass. However, there are supposedly enough of these to account for the extra mass needed to propel galaxies at such high rates. As opposed to the first argument, the dark matter stuff is dispersed throughout an entire system, and not concentrated in a lone location. This theory helps explain the "halo" effect (the bulge of dark matter) around a galaxy. The particles are scattered so dispersedly, and there has to be so many of them, that they create a bulge in space-time. However, like the first argument, this theory is also easily disproved. If the matter were scattered everywhere, and there was no central mass, then how are the galaxies rotating so fast--and around what? A non-centralized mass would not be able to hold a galaxy together at such high velocities; systems on the edge of the galaxy would go hurtling outward. We know too, that this is not the case. We can see the rim of our galaxy and distinguish the Sagittarius dwarf--a neighboring galaxy that is orbiting the Milky Way. Without some type of strong central pull (which we know exists because we've observed the Sagittarius dwarf slowly being ripped apart and absorbed by the Milky Way) the rim systems would just fly off into Never-Never-Land. So what in tarnation could possibly have the effects of both theories? Why, that's elementary my dear Watson. Simple logic leads to but one explanation: there is a strong central mass and also, trillions upon trillions of particles dispersed throughout the galaxy. The central mass provides for a galactic hub, and the particles cause the bulge. This way, systems can rotate around the hub without being sucked in, and galaxies in close proximity to other, larger ones are provided with enough neighboring mass to cause orbital status. Since we are well aware of these two effects, both arguments must be correct to some extent. It's the best of both worlds! The only remaining unknown is the exact percentage of dark central mass vs. dark dispersed mass.

Okay then, where is all this stuff--I mean where is it located? Ah, the question of where exactly is all the dark matter finally comes out. Well, allow us to explain. From the motions of the Magellanic Clouds (two satellite galaxies that orbit within the Milky Way's "halo") we can conclude that the "halo" of our own galaxy spans a distance of at least 300,000 light years. In fact, some of the objects located even further away, indicate that our "halo" may extend twice as far--600,000 light years. This is monstrously gargantuan! Traveling at the speed of light, it would take you six hundred thousand years to cross the Milky Way "halo". Our nearest neighboring spiral galaxy, Andromeda, lies two million light years away. Our "halo" may span a great distance to Andromeda and its "halo." Thus, it is probable to assume that other galaxies (beyond what we can measure) have "halo's" equally as big--if not bigger. Within individual galaxies, stars remain extremely far apart relative to their diameters. For example, from the sun to Alpha Centauri (our nearest neighbor) is humungous. 30,000,000 (thirty million) suns could fit between the two. In contrast, galaxies lie close together--relative to their diameters. Nearly all galaxies have a neighbor within a few diameters. Thus galaxies alter one another gravitationally, with the gravity caused by dark matter contributing to this interaction. In fact, even clusters of galaxies lie embedded in hefty quantities of dark matter. In any event, we are led to believe that dark matter associates with luminous matter, because the two frequently appear together. Yet, this inference may be biased in and of itself because luminous matter typically facilitates the discovery of dark matter. So you want a more specific location? Well buddy, there is no current straightforward answer; the stuff is everywhere in a galaxy--and that's all there is to it.