Dark Matter & Strings
One of String Theory's greatest accomplishments has been its ability to predict the so-called "graviton." This particle is the as-of-yet-undetected carrier of the gravitational force. Just as the other three fundamental forces in the universe (the Electromagnetic, Strong, and Weak forces) have particles that transmit their forces over space (the proton, the Z-boson, and the W-boson respectively) the graviton is the carrier particle of gravity. Not only does it predict this missing piece of the Standard Model, it gives it a wild property. While all the other particles in the universe are confined to our three-dimensional space, according to string theory the graviton isn't- it's allowed to roam the fourth dimension.
Our universe may be one of many universes in a four-dimensional multi-verse. Just like a slice of a hypercube if a cube, one slice of the multi-verse is our universe. Thus there could be universes that exist parallel to our universe in a real spatial fourth dimension. These special properties of the graviton hold water only if we assume the four-dimensional universe has negative curvature. Please refer to Figure 5.
Cosmologists believe that this might be the explanation of their field's biggest "pain in the rear"- dark matter. Why is it called dark matter? Only because scientists have not found any traces of it. Currently, our scientific knowledge of the universe can account for only 1 percent or so of the matter we know it to have. According to simple Newtonian equations, galaxies are not heavy enough to spin so quickly. The conclusion is made that there is matter (and a LOT of it) in galaxies that we cannot detect and scientists have no idea what it could be.
What does dark matter have to do with strings?
As stated in the first paragraph, string theory predicts the existence of the graviton that can move outside of our three-dimensional realm. Several million years after the Big Bang, our universe was a cloud of hot plasma. Pockets of this plasma slowly condensed by its own gravity, creating the infant stages of future galaxies. Figure 6 gives a two-dimensional analogy. As we see galaxies probably formed in tandem in parallel universes due to their mutual gravitational attraction. If this was the case with most galaxies then a galaxy in our universe will have a "partner" galaxy in each of its many parallel universes. These "partner" galaxies can only affect each other through gravity as the graviton is the only particle able to travel all four dimensions. The gravity of n-different "partner" galaxies are acting on every galaxy in our universe. According to our everyday experiences, mass is directly proportional to gravity.
Can it be there is a real fourth dimension?
Dark matter is matter in a universe in another dimension. Assuming that gravitons can traverse dimensions, galaxies in parallel dimensions would be relatively close to each other due to their gravitational pulls. Thus a galaxy in our dimension is not only be acted upon by its gravity but also the gravity of the galaxies in parallel universes. Thus there is enough mass to account for the galaxies rotational speed, just not in our universe. The same is true of the whole universe. If we include the masses of neighboring universes, then our universe should have enough gravity acting on it to have a Big Crunch (opposite the Big Bang)
Some other websites that discuss Dark Matter:
A site from Queen's University, Canada
A ThinkQuest Entry on Dark Matter