This week, I visited with some astrophysicists who are trying to detect dark matter in a laboratory on Earth. If they succeed (and are the first team to do so), it may well mean a trip to Stockholm. If they and all other groups searching for dark matter fail, it could pose a serious challenge to our current views of the Universe. It's a high stakes game, with a lot of pressure to be first and to be right.
Dark matter was first suggested by Fritz Zwicky, who was an astronomer with a, um, unique personality. Zwicky had many outlandish ideas well outside the mainstream. Many of these ideas were wrong, but many were right. One of Zwicky's ideas grew out of his studies of clusters of galaxies. The galaxies were moving faster than the gravity of individual galaxies would have predicted, so Zwicky posed that unseen matter was responsible for these fast motions. It took decades to show that this matter could not be normal matter (atoms and protons and electrons), but many lines of evidence have now shown that dark matter exists and cannot be normal matter.
But there remains a possibility that dark matter may not exist, but rather something else is going on. Perhaps our understanding of gravity is wrong, and gravity works differently on very large scales than on scales we can test in our laboratories. Perhaps the Universe folds back on itself in multiple dimensions and gravity from normal galaxies "leaks" through these folds while normal light cannot. (Don't worry if you don't understand it, I only have a vague idea myself.)
Assuming that dark matter does exist as a previously-unknown type of matter, it should be possible to detect. There are many proposed types of dark matter particles, with names such as axions, neutralinos, WIMPS, Kaluza-Klein particles, and many others. These particles react with normal matter mainly only by gravity, but very, very rarely they can actually bounce off of normal matter.
It is these rare bounces that Earth-based laboratories hope to detect. The physicists running these experiments cool a detector down to near absolute zero so that the atoms in the detector are barely moving. If a dark matter particle hits one of these atoms, the atom will move with a lot more vigor, and this extra movement can be detected with very precise measurements.
The problem is that there are lots of other, much more common events that cause the detector atoms to get excited and move around. Cosmic rays from outer space slam into these detectors with huge energies, heating many atoms up. Placing the detectors in mines deep underground protect them from cosmic rays. But normal particles called neutrinos can also penetrate the Earth and, on rare occasions, interact with the detector (in fact, many of these detectors double as neutrino detectors). Telling neutrino hits apart from dark matter hits is hard but not impossible. Another problem is radioactive decay. Even in a very pure detector, some atoms of radioactive material will be present, and radioactive decay also causes signals in the detector.
So, the entire game of dark matter detection is to look at hundreds of thousands of events and try to explain each one as a known process. The few unexplained events that remain are then considered to be dark matter candidates. Then even more tests are needed to see if these events are true dark matter. For example, as the Earth moves around the sun, the direction of dark matter particles should appear to change. So, if the average direction of the candidate dark matter events changes over the course of a year, then these events may be even more likely to be due to dark matter.
If one of the teams looking for dark matter actually finds it, and the experiment can be confirmed by other means, then we can finally be positive that dark matter exists. If the experiments are perfected yet fail to find dark matter, then many physicists will start to question if dark matter actually exists, or if some of the other, (currently) less-popular explanations like modified gravity may actually be correct.