So what could all this missing stuff be? To put it simply the two original competing theories were those of MACHOS and WIMPS. MACHOS are Massive Astrophysical Compact Halo Objects, put simply big things in our Galaxy that we can’t see directly. They could be brown dwarfs, black holes or neutron stars. While these don’t give out much optical radiation they can be detected by a novel technique called gravitational microlensing. In this a MACHO briefly gets in-front of a background star and it’s gravity focuses the star’s light towards Earth. This leads to the background star appearing slightly brighter. Studies of this effect such as the OGLE programme, seem to indicate that such objects could only make up a small fraction of dark matter in our Galaxy. Additionally, theorists can use the events shortly after the Big Bang and the observed abundances of light elements like Helium to set limits on how much baryonic (matter made out of protons, neutrons etc.) there is out there. It turns out that it’s only a small proportion of the implied dark matter density, so something else must be at work. By the way, the OGLE project didn’t just not detect a load of black holes, it’s also been very successful in finding planets around other stars.
So the main candidates left are the Weakly Interacting Massive Particles or WIMPS. If these are the main component of dark matter then we can constrain what their properties are without knowing exactly what they are. Firstly we don’t detect loads of interactions between these dark matter particles and ordinary baryonic matter, so they must interact weakly. Secondly we can tell something about how fast they move from how clumpy the Universe is. If the dark matter is slow-moving (aka Cold Dark Matter), then it will form lots of small clumps which can merge together to form galaxies. This implies there should be a lot of small galaxies. If the dark matter moves fast (aka Hot Dark Matter), close to the speed of light, then it should have a smooth distribution with very few clumps, this would lead to very few small galaxies. Scientists can do simulations of the evolution big chunks the universe, starting with some initial conditions and a particular type of dark matter. They let these run and see how the outputs compare with the distribution of matter we see in the Universe. These simulations indicate that Cold Dark Matter is a much better fit than Hot Dark Matter (although there are some details that still don’t quite fit).
So it looks like dark matter is made out of some sort of massive particle, that doesn’t move close to the speed of light and doesn’t interact with normal baryonic matter much.
PS The title of this post isn’t quite right, but I couldn’t forgive Emma for passing up an obvious title for yesterday’s post.