Dark matter

  When considering the universe we observe only what we can see. Nonetheless there are strong indications that there is something more. Suppose you look at how stars in the outskirts of a galaxy move. Since gravity decreases with distance one would expect that the stars would slow down as the distance to the galaxy center increases, but this is not what is seen: the speed of these outlying stars appears to be constant (see Fig. 8.19). This is explained by assuming that the galaxy is in fact surrounded by a mass of matter which emits no or very little light, the so-called dark matter. In fact, calculations show that if this hypothesis is correct, this kind of matter is the main ingredient of galaxies, and perhaps the whole universe; an illustration of the ``dark matter halo'' surrounding a typical galaxy is given in Fig. 8.20

  

Figure 8.19: Rotation curve for stars in the Andromeda galaxy. The velocity becomes constant far away from the center suggesting the presence of dark matter.

  

Figure 8.20: Illustration of the dark matter halo surrounding a typical galaxy.

What is this dark matter? No one knows! Is it perhaps a very large number of rocks, or planets? Is it something else? Or, maybe, is there a completely new effect which we interpret as dark matter while in reality there are new forces in action? The only recent answer is that there are strong indication that there are large numbers of planet-like objects in the vicinity of our galaxy. But these are not nearly enough to account for the whole effect. Many experiments are under way aiming at detecting the nature of dark matter (and it very existence).