From these data, we can learn a few important things. First, dark matter seems to act like particles, based on the way it collects and behaves under the influence of gravity. Also, dark matter is "cold", meaning that it doesn't move very rapidly; dark matter that is too warm will resist clumping together as much as we observe. That in turn places some limits on the lowest mass dark matter particles could have: if they're too light, they tend to move faster. Neutrinos, despite being extremely numerous and not affected by the electromagnetic force, are too light and therefore too "hot" to be dark matter.
To keep things cold, dark matter particles must not collide readily, either with each other or with ordinary matter. That's because collisions transfer energy, leading to higher speeds, and ultimately in warming up the dark matter. A lot of research is focused on testing how much dark matter can collide in particularly dense regions, such as near the centres of galaxies.
Diary of a WIMPy particle
Observational evidence may have ruled out two of the three non-gravitational ways that dark matter could interact with ordinary matter, but that still leaves one possibility: the weak force. In fact, several extensions to the Standard Model predict the existence of weakly-interacting particles, including a popular theory called supersymmetry (Susy, often pronounced SOOsee) – one of the cornerstones of string theory. The most promising dark matter candidate that encompasses all these possibilities is known as a Wimp: a weakly-interacting massive particle.
However, that's a big category. Worse still, models don't predict a specific mass for any Wimp candidate, leaving experimental physicists with ranges of possible values. (That's not a problem specific to Wimps, of course: the Higgs boson also lacked a specific mass prediction from theory.)
Despite all these issues, a variety of experiments and observations have helped trim down the Wimp mass range.
For example, the XENON100 experiment, located deep under the Gran Sasso mountain in Italy, contains a target of liquid xenon at -91C in a stainless steel cylinder. As with neutrino detectors, putting dark matter detectors underground helps prevent cosmic ray particles from interfering with the equipment. If a Wimp enters the cylinder and hits a xenon atom, the atom responds by losing electrons. Electrically charged plates at the ends of the cylinder push the electrons to the top, where they are recorded, along with any light the xenon gave off as a result of the collision. Because they interact so weakly with matter, each Wimp is unlikely to collide with anything, so the XENON100 team looks for single collisions. Anything more than that, and the collisions were probably another type of particle, such as a neutron.
In July 2012, the XENON100 research team announced that they had failed to find any Wimp signature above a certain mass limit, after operating for 225 days spread over 13 months. In particular, these data excluded several possible Wimp ranges from Susy, which is consistent with other disappointing results from particle colliders. The XENON100 results also rule out possible detections from other experiments, leaving dark-matter hunters gnashing their teeth.
However, the major dark matter detection experiments are upgrading, and new ones are in the works. The major goals in all of these efforts include reducing background signals (from cosmic rays and other particles) and increasing the sensitivity in mass ranges where various models predict Wimps may be hiding.
Exotic forces
If particle detectors cannot unmask dark matter’s identity, maybe astronomy can come to the rescue. Just as ordinary matter comes with antimatter partners, dark matter should have its counterpart. So detecting annihilation of dark matter/antimatter pairs in dense regions in space could offer some indirect evidence. Data from the orbiting Fermi Gamma Ray Observatory showed a tantalising signal coming from near the Milky Way's centre that could possibly have been from Wimp annihilation. However, the result dwindled into insignificance after further analysis. Similar observations of dwarf galaxies also failed to find clear dark-matter annihilation signatures.
