A Germanium puck from the experiment CoGeNT, designed to detect dark matter.
Dark matter has resisted all methods of detecting it. Initially it was thought that it might just be hidden ordinary matter that wasn't illuminated, such as black holes or cold dead stars. But this was excluded because such matter would also absorb radiation passing through it and this isn't what we observe. Dark matter appears to be a new form of matter which is totally transparent.
Many candidates for new types of matter with exotic names such as WIMPs (Weakly Interacting Massive Particles) and WISPs (Weakly Interacting Slim Particles) have been proposed for dark matter. Many experiments have been set up to try and detect dark matter, with no conclusive results yet.
Theoretical physicists have also proposed new theories that correct the known laws of gravity at large distances, without invoking new kinds of matter to account for the various observational discrepancies.
Time will tell what the solution of the dark matter conundrum is. With so many experiments running, it may not be long before we know.
The distribution of dark matter is much less dense than ordinary matter. The Earth's crust has a density of 3g/cm3 whereas for Dark Matter it is 0.0000000000000000000000005 g/cm3.
It isn't antimatter, because there would be radation produced on annihilation with normal matter.
Massive compact halo objects (MACHOs) are hidden heavy normal objects, such as black holes, neutron stars, faint old white dwarfs or brown dwarfs, that don't put out enough light for us to see them.
They cannot be black holes because the gravitational lensing would be observable, but a more plausibly candidate could be brown dwarfs.
A team called the "MACHO project" observed the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way, over a period of 6 years looking for gravitational "microlensing" events that could be associated with MACHOs wandering across the field of view between us and the LMC. They observed around 15 lensing events and thus set a limit of about 20% for the contribution to the dark matter in our galaxy from objects with mass less than half that of the Sun [Alcock et al., 2000]. This is not enough of an effect to account for all dark matter.
[The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. ]
Some form of cold dark matter (i.e. non-relativistic) is the most popular contestant.
There are two categories of experiments looking for dark matter: direct detection experiments that look for rare interactions between dark matter and ordinary matter, typically in deep underground laboratories to avoid interference from other sources such as cosmic rays, and indirect detection experiments that search for side-effects of dark matter, such as WIMP particles annihilated to produce gamma rays.
[Alcock et al., 2000] C. Alcock et al., "The MACHO Project: Microlensing Results from 5.7 Years of LMC Observations", Astrophys.J. 542 (2000) 281-307 arXiv:astro-ph/0001272.
Author: Tom Brown
Copyright: public domain
Date last modified: 13th Oct 2011
Peer-review status: Not yet peer-reviewed