Dark matter is believed to make up about 90% of the matter in the universe. Our primary evidence for it rests on the fact that large gravitational systems–galaxies, clusters of galaxies, and the universe’s large scale structure–do not move as we would expect from just taking into account the visible mass (mass due to stars and such).
The simplest model for dark matter is that it consists of elementary particles that interact with other matter mostly through gravitation, and possibly the weak force. This makes these particles very hard to detect (the neutrino also only interacts via the weak force. We only can detect its presence through very very rare interactions with heavy-water nuclei in giant vats of the stuff buried deep underground! Otherwise, we detect it purely through its absence).
Detection of the particles only through gravitation is good, but not entirely satisfying. Certainly, one would want another way to see that it’s out there. Well, according to this article:
we may well have started to see it. One possibility stems from the basic idea is that dark matter, like all other matter, has an “anti” version of itself. When antiparticles and particles collide, they release energy. Apparently, some models of dark matter predict that this will be seen as an increase in the amount of positrons (anti-electrons). Indeed, this is what PAMELA appears to be detecting.
The observation of alternative signatures for dark matter is a big deal. By learning more about what signatures it has, we learn more about what models of dark matter are right. Some of these models even stem from something called “supersymmetry”. Evidence for this would be a tremendous boost for string theory (but not sufficient to truly claim that strings has been proved).