In the first post of this blog, I briefly discussed how galaxy rotation curves provide evidence for the existence of dark matter - I didn't really said anything about what dark matter actually is. We've only said that it exists, that it has mass (i.e. it interacts with gravity), and that it doesn't interact with light like every day matter. The truth is, even though dark matter is 85% of the total matter in the universe, we don't know what it is because we've never seen it directly. That's one of the reasons we're looking for it. There are a number of theories for what dark matter might be, but for now I'll just talk about one of the most popular, the Weakly Interacting Massive Particle or WIMP (again, a cute name that is quite a literal description of a particle that has mass and interacts weakly).
There are four forces in nature. The first is gravity, by which mass attracts other mass. The second is electromagnetism between charges, such that like charges repel and opposite charges attract (electromagnetism is also the interaction between matter and light). The third force is the “strong” force, which holds protons and neutrons together. The final force, and the one of interest here, is the “weak” force, which is involved in nuclear reactions. The weak force is weak mainly because its range is very small. You have to be really, really close to something to interact weakly. For example, there is a very light particle called the neutrino that only interacts weakly (neutrinos are too light to constitute dark matter). Neutrinos are produced in nuclear reactions, including nuclear reactors. As the Sun is basically a giant nuclear reactor, it emits neutrinos all the time - 60 billion solar neutrinos go through each one of our fingernails every second, but they just don’t hit anything; basically, because neutrinos only interact weakly, we are transparent to them.
A heavy particle that interacts weakly, or a WIMP, is exactly the kind of thing that could be the dark matter - we simply wouldn't have observed it before because the weak interaction is so rare.