The predicted model of galactic rotation based on what we can see (line A) versus what is actually observed (line B); courtesy of Wikipedia.
We have a huge problem. Like a we-seem-to-be-missing-95%-of-the-universe kind of problem. If you look at the way that galaxies are constructed you will notice that they are very bright in the center and tend to get dimmer as you travel outward, indicating there must be more mass in the center than the outside. This would mean that the center of the galaxy should be spinning much faster than the outermost edges (represented by line A on the graph above), but that’s not what actually happens. What we observe is that all points in a galaxy seem to rotate at about the same angular velocity (line B), which suggests that galaxies are actually in the shapes of discs with constant densities throughout. We can’t see the matter that’s supposedly causing this, so… what’s happening?
The most commonly accepted theory is that the matter we can’t see is actually dark matter, a type of matter that acts like any other particle except for the fact that it doesn’t emit any detectable radiation. Under this theory, there are two main types of dark matter: baryonic and non-baryonic dark matter. Baryonic dark matter would be any kind of normal, everyday matter that is simply not being exposed to enough radiation for us to detect; for example, there could be a brown dwarf that is not a part of any solar system, just sitting out on its own in space. Because it has so little light to reflect, it would be all but imperceptible to us, technically rendering it dark matter. Black holes would qualify as well, as they emit no radiation.
The second, more theoretical type of dark matter is non-baryonic matter, which is a new type of matter entirely and is not composed of the traditional atoms as we know them. These types of particles would only interact with the Weak and Gravitational forces, meaning that it would not interact with any form of radiations and would therefore be unseeable by any direct means. The most convenient way to detect matter that is truly invisible would be to look at the gravitational effects around it, such as abnormal orbits of other bodies or the gravitational lensing effect on light.
While dark matter is still only a theoretical solution to the missing mass problem, it does offer a convenient explanation and the physical evidence we have does seem to support the theory. The major problem will be finding conclusive evidence of non-baryonic matter, and it is believed that the discovery of the Higgs boson will be a major step toward that goal.
