Dark matter has been a polarizing subject. It hasn't been detected, the name implied it was a mystery, and it started out as an explanation for the apparent extra but invisible mass in galaxies. But the evidence that something unknown is out there has become rather encompassing, appearing in the cosmic microwave background, galaxy clusters, and even apparently empty space. Even if dark matter doesn't exist, something will have to fill a whole bunch of gaps at many different scales of the Universe. Nevertheless, it is a placeholder concept, a hole in our knowledge that we can feel the shape of but haven't yet managed to capture in the spotlight.

So, what is dark matter? One possible answer is a modified theory of gravity, but the favorite proposal at the moment is a class of particles called weakly interacting massive particles (WIMPs). The distinguishing feature of WIMPs is that they are not dark at all—instead, they interact so rarely with normal matter that our current instrumentation is blind to their effects. The fact that these particles interact at all is probably one of the main reasons that physicists prefer the WIMP explanation: if WIMPs exist, we could build an instrument to see them.

And build them we have. Two teams have now claimed to have detected dark matter particles. But last month the XENON100 team published its own data, claiming that the earlier results are bunk and dark matter cannot possibly have been detected.

The thing about dark matter detectors is that you can't just grab one off the shelf from Radio Shack. And to make matters worse, no one is quite sure what they are trying to detect, so there's a wide variety in instrument design, with each design optimized for detecting dark matter particles that fall within a certain parameter range. This makes sense, since it covers more possibilities, but it also gives you a fit of nerves as you try to figure out who's looking at what.

Building a dark matter detector

If you want to detect dark matter, the first thing you need to do is find a really dark place: the bottom of a deep mine shaft is a favorite. Not only is light blocked, but so are most high-energy particles from space. This leaves you a relatively clean background in which to see any dark matter, which should pass through nicely, given that it doesn't interact with much.

To get a WIMP's attention, you stick a nice pure liquid at the bottom of the shaft. You want it to be pure so that, should a WIMP decide to get aggressive and punch an atom, it only has one choice. When a WIMP collides with an atom, the atom becomes excited and releases a photon.

After that, it is a case of sorting out a signal from all the events due to cosmic rays, neutrinos, background radiation decay, and other assorted nasty interfering background. This is done in two ways: given an energy range, certain decay processes are forbidden—there is not enough energy for them to run, or they don't conserve momentum—so some signals can be directly eliminated. The rest have to be filtered on a statistical basis. And this relies on having excellent models of all the processes that can generate a signal in your detector. Verifying these models and eliminating potential errors consumes vast amounts of time, neurons, and, I suspect, beer money.

So when the DAMA/LIBRA consortium reported that they had a potential dark matter signal, everyone was a bit cautious. Well, actually, very cautious. They had cleverly used the motion of the Earth relative to the galaxy's center of mass motion to find a signal that varied seasonally. When the Earth is moving in the same direction as the galaxy, it passes through more dark matter (because dark matter doesn't rotate with the galaxy), generating a larger signal in the detector. The signal was robust—break out the champagne. OK, not quite.

How long can you wait impatiently?

Fast forward two years and, finally, a second detector sees the annual variation that may be a signature of dark matter. Two years is a long time, but these are complicated experiments, so maybe it's all good?

Not so fast, says the XENON10 collaboration, which has also been searching for the annual variation in dark matter. Their detector uses Xenon as the fluid, which is not so well suited to find particles in the energy range that the DAMA/LIBRA collaboration had been designed for. Cleverly, instead of redesigning their instruments, the XENON10 folk redesigned the algorithms that filter out events so that they were optimized to look specifically in the right energy range.

When that was done, they estimated that they had a sensitivity that was sufficient to see dark matter at a level that was a factor of ten smaller than necessary to replicate the DAMA/LIBRA results. But the results from the XENON10 data are incompatible with those from DAMA/LIBRA. They found absolutely zip.

Both teams are, of course, sure they have it right; still, in the end someone must have fallen victim to a systematic error in their analysis. The end result is that now there are three teams of scientists all puckered up—buy a team member a beer in commiseration should you run across one.

Physical Review Letters, 2011, DOI: 10.1103/PhysRevLett.107.051301