We haven't seen dark matter yet. We haven't, right? Sitting in a plenary talk at the APS April meeting today I started to have my doubts.
Dan Hooper from Fermilab gave a great overview discussion of the attempts to detect dark matter covering the three major techniques: direct detection, where you see dark matter particles collide with nuclei; indirect detection, where you use telescopes to observe the gamma rays produced by dark matter annihilating; and collider detection, where you create the dark matter in something like the Tevatron or the Large Hadron Collider.
In the discussion, Hooper pointed out the various experiments which have seen hints of dark matter. Of these, only one is claiming to have definitively seen dark matter--the DAMA collaboration, which looks for seasonal variance in a signal representing the amount of dark matter hitting the Earth. Nobody doubts they have seen a signal, there is just debate about whether it is due to dark matter or some other, so far unexplained, effect.
However, there are other experiments which have seen hints of something. That includes CoGeNT, an underground experiment, and the Fermi Gamma-ray Space Telescope. There's even an intriguing hint of how the Tevatron might shed light on the dark matter (see our next post). But where it gets really interesting is that the properties of dark matter in each of these cases is the same. They all seem to have a mass of about 5-10 GeV (a proton is about 1 GeV). Also, their cross sections--the likelihood that they will collide with something--is about the same in each case. The next piece of evidence that would help is if CoGeNT sees a seasonal variation in its results--that data is expected to be released soon.
There is also some suspicions of dark matter observations from the PAMELA satellite but that would not be consistent with these other measurements and has a few potential explanations that don't need dark matter. Then there are experiments like XENON100 which claim to rule out the dark matter explanation for DAMA and CoGeNT.
So what is the situation? We have one claim of dark matter, which hasn't been replicated in other experiments, and two or three other hints that might be dark matter, all coinciding in mass and cross section. Then there are experiments which claim to rule out some of those experimental results. Any of the experimental results by itself can be explained away but taken in concert, the picture looks a bit different.
Just what would discovery of dark matter look like? Well, with all these different experiments looking for it, you'd expect to start to see hints of its existence in them potentially before any could claim discovery. In other words, it would look just like this.
We can't say that we've seen dark matter but might we be able to