Today, the scientific community generally accepts that 95% of the universe is made up of stuff that we’ve never seen before and do not understand; the chart at the right shows the composition of our universe as measured by the NASA/WMAP satellite (the subject of a future series of posts). We think about a quarter of this unknown density is in the form of dark matter. Although the phrase “dark matter” sounds sci-fi and romantic, it’s actually a simple description of what we think it is – matter that does not interact in the usual way with light, unlike the ordinary matter that makes up the Earth, people, the Sun, and everything else that we can see in the sky at night (the remaining 5%).
There's a lot of circumstantial evidence for the existence of dark matter (rotation curves, gravitational lensing, the cosmic microwave background, etc.), and I hope to go through much of it in future posts. For now though, I will briefly describe the simplest piece of evidence, the rotation of galaxies.
Because of gravity, galaxies orbit about their center, just as the Earth orbits around the Sun – we’ve all probably seen nice pictures of spiral galaxies (like this picture of Andromeda), which are spirals because of the rotation. We can figure out the speed of the rotation using the Doppler effect, which says that the frequency of an observed wave will be shifted depending on the relative speed of the source and observer. A familiar example of the Doppler effect is that the frequency and pitch of a police siren will change to a listener on the sidewalk as the police car passes by (A cartoon illustration).
Since light is also a wave, the frequency and wavelength of light coming from a distant galaxy will also be shifted if the galaxy is moving – this is known as “red shift.” In a rotating galaxy, one side is spinning away from us while another side is spinning towards us, and the difference in shift from one side to the other can tell us how fast the galaxy is spinning. Simple Newtonian mechanics can predict the speed of rotation at a point in the galaxy as a function of mass and radius, and for a constant mass, the speed of rotation should decrease with increasing radius (this makes sense intuitively – the force of gravity decreases with increasing distance, so if the mass is held fixed but the distance increases, there just isn’t as much force to pull the galaxy around). Therefore, if all the matter in the galaxy were in the central, bright part of the galaxy (the part that interacts with light), we would expect the speed of rotation to decrease away from the central, bright part of the galaxy. In fact, the speed of rotation stays constant out to very large radii, much farther than the extent of the bright part of the galaxy. Therefore, there must be matter in the galaxy that we cannot see to pull the galaxy around – this is “dark matter.”
The dashed line is what we would expect to see if all the matter were in the bright part of the galaxy. The solid line is what we actually observe