So what is Dark Matter?
Presumably there must be some kind of exotic particles that constitute Dark Matter (DM). The ‘Standard Model’ of particle physics looks like this:
It turns out there are candidates for DM in this model. ‘Neutrinos (the ‘e’, ‘μ’ and ‘τ’ in the leptons group) are DM candidates. Millions of neutrinos pass through the Earth and through our bodies every second, only very rarely interacting with matter. However, neutrinos have an extremely small mass and there are not nearly enough of them to account for the amount of DM required. One group of researchers postulates ‘sterile neutrinos’ which supposedly only interact with other neutrinos, arising when an ordinary neutrino morphs into a sterile neutrino. These results are highly contentious in the community, so neutrinos may only offer a partial explanation.
Another theoretical possibility is called a ‘Massive Compact Halo Objects’ (MACHO), a body composed of normal matter whilst emitting little or no radiation. Possible MACHOs include black holes, neutron stars, red dwarf stars and brown dwarf stars, or even planets not associated with any stars. These would be very faint and emit mainly at infra-red wavelengths rather than optical. Some, not completely conclusive observational evidence for MACHOs has been obtained via gravitational micro-lensing observations. Future observations by the upcoming James Webb Space Telescope, which will observe in the infra-red, may detect MACHOs, but there is still a problem. Theoretical studies indicate MACHOs cannot comprise more that 20% of the required dark matter. Add to that the 3% of normal matter we can see, and we still have the question “where is the other 77%?”
Another DM candidate is a theoretical, non-baryonic particle named ‘Weakly Interacting Massive particle (WIMP). The characteristics of a WIMP are framed such that if they exist it would answer the question as to what DM is. The theory is that WIMPs ought to interact very weakly with baryonic matter. The inferred distribution of dark matter in our galaxy (i.e. the DM halo) shows a considerable contribution in our location, so as we move through space, we ought to pass through much DM. If DM is made of WIMPs, then we could directly detect the rare interactions between WIMPs and ordinary matter. The existence of WIMPs is allowed under an extension of the standard model of elementary particles called supersymmetry. The first problem with WIMPs is that supersymmetry theory has no observational basis. And the second snag; nobody has detected a WIMP.
The last current theory for DM postulates particles named Axions. As with WIMPs, the properties of Axions are framed such that they would account for DM. Because of these properties, axions would interact only minimally with ordinary matter. Axions are predicted to be electrically neutral, have very small mass and very low interaction cross-sections for the strong and weak nuclear forces. This would require modifications to Maxwell’s Equations. Axions would also change to and from photons in magnetic fields. Quite a wish list!
Current physics assumes gravity has always acted as it does now; acts the same everywhere; and under all conditions. Suppose that isn’t the case? The leading –though by no means widely accepted - alternative theory to DM is Modified Newtonian Dynamics (MOND) which postulates that under conditions of low acceleration, gravity behaves differently. It also asserts that the inverse square law, while being true over comparatively small ranges such as the solar system, is not applicable over galactic scales. While MOND appears to account for the motions of galaxies without the need for DM, it does not account well for the observed motions within galaxy clusters – reminding us of Fritz Zwicky’s 1933 DM conclusions.
MOND also flies right in the face of Einsteins General Relativity, which has passed every experimental test that has been thrown at it since 1917.
Most physicists believe DM exists. We do know what DM does. We have little idea about what DM is. Current explanations involve serious modifications of the Standard Model of particle physics, or serious modifications to General Relativity, maybe even both. It’s uncomfortable to think that we don’t know what most of the matter in the universe is. It’s an interesting time to be involved in astrophysics.
The author wishes to acknowledge the assistance of Bob Merritt in the preparation of this article.