Here is what is really troubling (or perhaps exciting, depending on how you look at it) about dark matter. Kinematic observations of galaxies and galaxy clusters tell us that if Newtonian gravitational theory is correct (and this might be the place to mention that relativistic effects are negligible for this situation, so we really are dealing with Newtonian gravity), then there has to be a lot of extra mass beyond what we can see directly in these galaxies. Of course, we could (and people have) come up with some sort of modified Newtonian dynamics that would explain the observations without dark matter, but in general we like to avoid introducing new physics for every new phenomenon.

On the other hand, suppose there really is dark matter out there. There's nothing spooky about dark matter; we ourselves are made out of it (Dark Matter? You're soaking in it!). However, we're talking about a lot of dark matter here. The dark matter required is enough to account for Omega of about 0.2. Now, abundances of the light elements place restrictions on the amount of baryons you can have at the time of primodrial nucleosynthesis, and here's the punchline; in order to get the light element abundances right, baryons cannot account for more than about Omega of 0.1. That means that if there really is as much dark matter as the observations suggest, then much of it, perhaps even most of it, must be made up of exotic matter.

In other words, no matter what we do, some sort of new physics seems necessary in order to resolve the dark matter problem.

There are currently two theories for what dark matter is. The first is the MACHO theory. MACHO stands for Massive Compact Halo Object. This basically means there are large objects orbit on the outskirts of the milky way. These are large objects which weren't big enough to become stars. They are probably about the mass of Jupiter. We know that at least some MACHO's exist by the way they lense or bend the light from a distant star. Although we have observed lensing, it doesn't occur frequently enough to account for all the mass that needs to be in the outer halo.

The other dark matter theory is for WIMPS. A WIMP is a Weakly Interacting Massive Particle. In this case massive means massive on an atomic scale. These particles have a tendency to not interact with other particles. They also cannot radiate away energy. Because of this they can't lose energy easily and orbit at great distances. This would explain the extra mass needed to sustain the rotation of galaxies.

It will be a long time before we have conclusive evidence as to what dark matter is and whether or not it exists.
Novel written by Garfield Reeves-Stevens, published in 1990. Tagline: "A Novel of Genius, Madness, and Murder."

This story is a disturbing but effective mix of horror, mystery, and a good deal of quantum theory. It's written well enough that you don't have to actually know anything about quantum theory to understand the progression of the story. It is fiction, but uses just enough scientific fact to be readable without seeming totally absurd.

The plot involves a serial killer, a homicide detective, a team of American Physicists, and a cat that isn't quite a cat anymore.

"He held up his multi-dimentional fist and called forth the seething rage of the Water, commanded the Earth to hold its ground, caused the Air to move between them, brought down the Fire of the sun to fuel all life, creating the universe entire with his act of will And through it all, the echo of his laughter rang, filling all of spacetime with the enormity of the joke he had seen: Three thousand years after the Greeks and their four basic elements, the modern world had come to this, to this - the same four basic forces.

Three thousand years to go from four to four. Cross' mind reeled with the absurdity. Why had no one seen it before? He held the Fire of the ancients in one hand, matched by the radioactive fire of the Weak Nuclear Force in the other. He cupped the Water in one hand, and the ebb and flow of electromagnatism in the other. His one fist closed on the solidity of the Earth, his other felt the binding strength of the Strong Nuclear Force.

And he juggled everything in the all-encompassing Air - the absolute context in which the other three elements existed for the ancients.

And he juggled everything in the all-encompassing Gravity - he saw it clearly now - the absolute context in which the other three forces existed for all time."

-Garfield Reeves-Stevens, Dark Matter

Garfield Reeves-Stevens is also the author of three novels originally published in Canada: Bloodshift, Dreamland, and Children of the Shroud. His American debut was Nighteyes, in 1989.

Dark Matter is an RPG game world published by TSR (now Wizards of the Coast). It is a modern day setting dealing with conspiracy lore, the paranormal, the occult, UFOs and extraterrestrials, etc.

From the back cover:

Think back to every story of alien visitors, psychic powers, occult lore, unexplained miracles, strange cults, and secret societies that you've ever heard... then imagine if they were all true, and your job was to investigate them.

This new campaign setting for the Alternity science fiction roleplaying game rips away the comforting veils of ignorance, exposing the world's corruption and rot for all to see. Devious organizations scheme for world domination, otherworldly forces infiltrate our power structures, and creatures from our nightmares lurk in the shadows. Working for a clandestine organization called the Hoffmann Institute, heroes explore hidden mysteries while eluding forces--both human and alien--that scheme to control the truth.

the sun has a sister
a dark little petal
of a white spiral rose

her heart never exploded
like her effervescent brother
a trillion miles away

she looms through the comets
invisible, planet eater
silent in dreams

she knows what she is -
dark matter
for crushing the universe again

This is original work
Look up at the night sky - what do you see? All but a tiny fraction of the light that you see (and that being the light reflected off of planets and moons) is directly from stars. This is what astronomers study and cosmologists measure. However, the sum total mass of what we can see appears to only be a small fraction of the mass out there.

There are two ways to detect matter in the cosmos - seeing it directly (as with the light) or observing its effect upon light. Direct observation is the oldest and has been performed since the dawn of astronomy. More advanced telescopes have enabled astronomers to see smaller objects and objects further away - be it with the Hubble Space Telescope or with the giant radar dish of Arecibo. And yet, there are some things still to dim to see.

Recently, extra-solar planets have been discovered - not through direct observation, but rather by observing the gravitational tug of a planet (that which cannot be seen today) on a star (something that can be seen).

This brings us to dark matter. In the 1930s astronomers (primarily the Swiss astronomer Fritz Zwicky) were looking at galaxies and found that the vast majority of galaxies are clumped together in great clusters and along chains rather than stand in vast empty expanses. Looking at both the velocities galaxies traveling within the cluster and the mass of the galaxies, they were in for a shock - the velocities were off by an order or two of magnitude from what was expected with the estimated mass of the galaxies - traveling 10x to 100x faster than expected.

While good evidence of something strange, the velocity of galaxies with respect to clusters of galaxies has problems. It is difficult to determine if a galaxy is gravitationally bound to the cluster, sailing through, or is just a foreground galaxy.

Stronger evidence for dark matter came in the 1970s when the velocity of stars within a galaxy were examined. Just as planets spin around the star, so do stars spin around the center of the galaxy. These follow clearly from Kepler's Laws in which the rotational velocity of a body around the center depends only upon the distance to the center and the mass contained within the orbit.

With planets, this is clearly seen - The Earth takes 365 days to go around the sun, while Mars takes 687, Jupiter takes 4332, and Mercury zips around the sun every 88 days. With a galaxy, one would expect the stars near the center of the galaxy to be zipping around rapidly, while the stars at the edge to be moving very slowly. This is not the case - it turns out that the stars at the edges of the galaxy are moving much faster than they should be if the vast majority of the mass of a galaxy is at the center (like it appears to be). One expects the orbital velocity of stars to drop off rather than level off.

    Expected          |     Observed
s ^                   | s ^
p | .                 | p |      . . . .
e |  .                | e |   .
e |   .               | e |  .
d |     .             | d | . 
  |       . . .       |   | .
  +-------------->    |   +-------------->
    radius                  radius

While the spiral galaxies are the best evidence to date of dark matter, there is evidence also with star velocity in globular clusters, elliptical and dwarf galaxies (such as Sagittarius DEG).

The amount of mass in the universe is of interest to cosmologists in trying to determine the ultimate fate of the universe. Cosmologists have a value called Omega that relates to the total density of mass within the universe. If Omega is greater than 1, the universe is open - if it is less than one it is flat. At the precise balance of exactly 1, the universe is balanced between the two.

Taking all the visible matter in the universe into account gives an Omega value of 0.05 - stating that the universe should be flying apart. It more closely appears that the omega is close to 1, if not exactly one (though some put it at 0.4). This gives us the range from 80% - 95% of the mass in the universe is not seen.

There are many theories as to what dark matter is - some ordinary, some exotic.

While the earth isn't that massive, Jupiter gets up there - one theory is that of a lot of Jupiters could be some portion of the dark matter. Unfortunately there are some problems with this. The first is that planets only form around stars and then appear to only make a few percent of the mass of a solar system. Furthermore, when one looks at the Big Bang Nucleosynthesis it states that the vast majority of the normal matter (baryonic matter) within the universe is in the form of Hydrogen and Helium - there just isn't enough matter for planets to make up the dark matter. Furthermore, the BBN predicts the total amount of baryonic matter in the universe can only make Omega get up to about 0.2 (0.2 is the high estimate - it is often placed at 0.1).
Burned out and non-luminous stars
From Jupiter on up there is a range of almost stars called Brown Dwarfs (about 10x more massive than Jupiter). While these stars are not massive enough to produce light, they do exist hand have been detected. Likewise there are the cores of stars called White Dwarfs. These two stars are expected to be the most common stars in the universe and are barely detectable when nearby. However, the BBN still predicts that there isn't enough normal matter in the universe to make up for the Omega. These are often called MACHOs standing for Massive Compact Halo Objects.
Black Holes
Black holes range from stellar mass to super-massive. These are often invoked as candidates for dark matter. Quiet super-massive black holes (as opposed to their noisier kin in galactic centers and quasars) are very hard to detect, emitting only a tiny fraction (10^-11 of the rest mass).
At the other end of the spectrum is the exotic very small matter that may permeate the universe. While not as easily understood, this appears to be the case for what dark matter is - the Big Bang Nucleosynthesis indicates that normal matter can only account for Omega of to 0.2 - only accounting for at most 50% of the lowest estimate for Omega and only 20% of the critical value.
Neutrinos (hot dark matter)
Neutrinos were produced in staggering numbers in the early universe to the point at which the universe is thought to be saturated with them. Neutrinos are believed to account for 1.7 Kelvin of the 2.7 Kelvin cosmic background radiation. If neutrinos have mass (which it is believed they do) they can make a substantial impact on the value of Omega. However, these particles cannot account for all the dark matter - dwarf galaxies seem to have a much higher mass density than can be accounted for by neutrinos which would necessitate at least two different kinds of dark matter - one for low mass galaxies and another for more classical galaxies.
WIMPs (cold dark matter)
WIMPs make up a wide array of dark matter candidates. These particles have two qualities to them - Weakly Interacting and Massive Particles (thus the name wimps). These particles were believed to be created in the early universe (thus the statement "It takes GUTs to have WIMPs" refering to the Grand Unified Theory).
There are other postulated particles that are allowed by the current theories - photinos which are cousin to photons but have a mass of 10x to 100x of a proton. Axions (a rather light particle theorized by quantum chromodynamics) and quark nuggets made up of strange quarks.

By no means do all astronomers agree with dark matter - which is still only a hypothesis (though a very strong one). One very distinct possibility is that we do not understand gravity properly. This lack of understanding can range from a non-zero Cosmological Constant to a modification of how gravity works at very low acceleration (more about this can be read at MOND and the Pioneer space probe acceleration anomaly).

The question is - what is it that accounts for what we see. Until a better theory comes along, Dark matter is one of the better explanations for why the universe is the way it is.

Recent developments in theoretical physics have generated a new hypothesis for the origin of dark matter.

It has long been known that there are 4 basic forces that generate all matter and energy. weak nuclear force, strong nuclear force, electromagnetic force, and the gravitational force. Gravity stands out in that it is far weaker than any of the other three fundamental forces. This may be due to gravity propogating into other dimensions outside of our basic four.

The recent work of Paul Townsend, professor at the Department of Applied Mathmatics and Theoretical Physics at Cambridge, developed the overspecific superstring theory into the new p-brane theory (also see M-theory). From this it has been hypothesized that we humans live in a universe that is a p-brane, in other words it has more than four dimensions. The reason that we do not see these other dimensions is because they may be curled up really really small. Imagine a really thin piece of paper. It would look to be 2-dimensional unless you looked really closely and then you could see its third dimension curled up tightly.

Shadow Branes
This idea of a p-brane universe with multiple dimensions is intruguing because it is experimentally testable in the next generation particle accelerators, and it may explain dark matter. Famous physicist Stephen Hawking hypothesizes about the existence of a nearby "shadow" brane, similar to our own in his book The Universe in a Nutshell. Gravity would be able to travel through other dimensions onto this brane while other particles and energies would be limited to our own brane. Dark matter then could be mass residing on the nearby brane. "In the brane world scenario, planets may orbit a dark mass on a shadow brane because the gravitational force propogates into the extra dimensions" (Hawking, 185). The way in which gravity propogates into the extra dimensions would account for the weakness of gravity compared to the other forces.

The shadow brane hypothesis would explain why we haven't been able to detect dark matter other than by it's gravitational influence. In large galaxies the outlying stars shouldn't be travelling as fast as they are from the experimentally determined amount of mass in their galaxy. There is missing mass evaluated as dark matter which may reside on a shadow brane. "We would not see a shadow galaxy on a shadow brane because light would not propagate through the extra dimensions. But gravity would, so the rotation of our galaxy would be affected by dark matter, matter we cannot see" (Hawking, 188).

Hawking, Stephen. The Universe in a Nutshell (Bantam Doubleday Dell Pub, November 6, 2001.)

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