Dark matter is probably something you have heard about, although odds are that you do not know exactly what it is. (Editor interjects: mainly because nobody does...). To explain it simply, think of the universe as a pie: about 4.9% of it is made up of ordinary matter (kind of like the pastry on top). That is the kind of matter made up of atoms those things you learn about in chemistry. That is to say that everything you learn about in chemistry only makes up 4.9% of the universe (further evidence, as if it were needed, of the superiority of physics). That number includes everything you know and love: your family, your friends, trees, plants, the atmosphere, the Earth – all of us in that tiny little number. The most incredible thing about humans is just how insignificant we really are.
Going back to the pie, our universe is also made up of 26.8% dark matter (kind of like the pastry around the base). The rest is something known as dark energy, analogous to the filling the
main body if you will (68.3%). So what is dark matter? Well, we do not actually know, but we do know some things about it. Physicists believe that it is made up from weakly interacting massive particles (WIMPs for short). The “weakly interacting” part of WIMPs means that, at the beginning of the universe, when the universe was very hot and dense, dark matter was able to turn into lighter particles by decaying (through annihilation). The fact that WIMPs are massive means that they create a gravitational field, giving them what is actually quite a large mass. But there is one minor problem: we, err... cannot seem to find these WIMPs.
Researchers at CERN in Geneva are currently searching for these WIMPs as they represent one of the largest areas of interest in physics today. So why do we think that dark matter exists?
Think for a moment about our Solar System, with the Sun in the middle and all the planets rotating around it. The planets orbit around the Sun because of a force we call gravity. Gravity is an incredibly weak force associated with the presence of mass: the Earth and Sun orbit their common centre of mass (which is basically inside the Sun because it is so big!) so to all intents and purposes we orbit the Sun, and the Moon orbits the Earth because again the centre of mass of the system is within the Earth. Physicists are also able to calculate the speeds of these objects in orbit using gravity (in fact we rely on what is known as Gauss’s law which tells us that a body behaves as if it were orbiting a point mass at the centre of mass of all of the objects closer to the centre of a galaxy than the body itself). However, when Physicists add up all the mass, such as stars, planets, gases, and dust in our galaxies, it appears that the galaxies are rotating much too fast for the amount of stuff actually within them. So where does this invisible source of mass come from? In the way physicists are wont to do, we gave it a name: “dark matter”.
Dark matter is dark because it does not react with photons (light) in the way in which you would expect. This, along with the fact that dark matter also does not interact with matter very much, makes the WIMPs extremely difficult to find (they may be all around us right now!). There is, however, one way to visually see where dark matter is. This is through a phenomenon called gravitational lensing. As the dark matter has such a large mass, it can distort the light of an object behind it through gravity. In other words, the gravity of dark matter can bend light around where it is.
I hope this explains what dark matter is to a small degree, and why it is called dark matter. At the moment dark matter is only a suggestion and is difficult to prove beyond reasonable doubt until the search for WIMPs is over. However, with the large hadron collider at CERN resuming its work last year, I think it is only a matter of time.
Image used under a Creative Commons license: https://commons.wikimedia.org/wiki/File:No_WIMPS_here_-_entrance_to_Boulby_Mine_-_geograph.org.uk_-_1626186.jpg.