What Can The (Dark) Matter Be?

What better place to look for dark matter than down a mine shaft? A research team from the University of Florida have spent nine years monitoring for any signs of the elusive stuff using germanium and silicon detectors cooled down to a fraction of a degree above absolute zero. And the result? A couple of maybes and a gritty determination to keep looking. 

The case for dark matter can be appreciated by considering the solar system where, to stay in orbit around the Sun, Mercury has to move at 48 kilometers a second, while distant Neptune can move at a leisurely 5 kilometers a second. Surprisingly, this principle doesn’t apply in the Milky Way or in other galaxies we have observed.  Broadly speaking, you can find stuff in the outer parts of a spiral galaxy that is moving just as fast as stuff that is close in to the galactic centre. This is puzzling, particularly since there doesn’t seem to be enough gravity in the system to hold onto the rapidly orbiting stuff in the outer parts – which should just fly off into space. 

So, we need more gravity to explain how galaxies rotate and stay together – which means we need more mass than we can observe – and this is why we invoke dark matter. Invoking dark matter also helps to explain why galaxy clusters stay together and explains large scale gravitational lensing effects, such as can be seen in the Bullet Cluster (pictured above). 

Computer modeling suggests that galaxies may have dark matter halos, but they also have dark matter distributed throughout their structure – and taken together, all this dark matter represents up to 90% of a galaxy’s total mass. 

Current thinking is that a small component of dark matter is baryonic, meaning stuff that is composed of protons and neutrons – in the form of cold gas as well as dense, non-radiant objects such black holes, neutron stars, brown dwarfs and orphaned planets (traditionally known as Massive Astrophysical Compact Halo Objects – or MACHOs). 

But it doesn’t seem that there is nearly enough dark baryonic matter to account for the circumstantial effects of dark matter. Hence the conclusion that most dark matter must be non-baryonic, in the form of Weakly Interacting Massive Particles (or WIMPs). 

By inference, WIMPS are transparent and non-reflective at all wavelengths and probably don’t carry a charge. Neutrinos, which are produced in abundance from the fusion reactions of stars, would fit the bill nicely except they don’t have enough mass. The currently most favored WIMP candidate is a neutralino, a hypothetical particle predicted by supersymmetry theory. 

The second Cryogenic Dark Matter Search Experiment (or CDMS II) runs deep underground in the Soudan iron mine in Minnesota, situated there so it should only intercept particles that can penetrate that deeply underground. The CDMS II solid crystal detectors seek ionization and phonon events which can be used to distinguish between electron interactions – and nuclear interactions. It is assumed that a dark matter WIMP particle will ignore electrons, but potentially interact with (i.e. bounce off) a nucleus. 

Two possible events have been reported  by the University of Florida team, who acknowledge their findings cannot be considered statistically significant, but may at least give some scope and direction to further research.

By indicating just how difficult to directly detect (i.e. just how ‘dark’) WIMPs really are – the CDMS II findings indicate the sensitivity of the detectors needs to bumped up a notch.