Dark Matter - What We’re Missing

Dark matter sounds exactly like what it is: elusive and fantastic. Dark matter was first hypothesized to exist by Vera Rubin, and since then the search for it has expanded to dominate almost all particle physics research. In 1970, while observing redshifts of galaxies to measure their rotations, Vera Rubin noticed something horribly off with the speeds of these rotating spiral galaxies. From what she could see, the galaxies were spinning much faster than they should have been able to with the mass they had. Somehow, they weren’t ripping themselves apart. She came to the conclusion that what she could observe, what she called “luminous mass,” did not comprise all of the mass in the galaxy. She called this unknown substance “dark matter,” as it does not interact at all with light, but still has mass and provides gravity.

The question that particle physicists have been wondering since Rubin’s discovery has been: how do you see something that can’t be seen? To start with, physicists have tried to get other theories to explain dark matter, so they can at least try to define how the particle acts. The Standard Model does not account for any sorts of particles that could form dark matter. To make matters worse, two opposing theories postulate the existences of two different types of dark matter, “hot” and “cold.” Hot dark matter would be very small and travelling at nearly the speed of light out of the Big Bang, as researched by Yakov Borisovich Zel'dovich, while cold dark matter would be much more massive and slow, as researched by James Peebles. It is now held that for galaxies to form after the Big Bang, both hot and cold dark matter must have been present.

Physicists have examined many candidates over the years to see if they fit the role of dark matter, even delving into physics beyond the standard model. The first candidate for the extra mass in the universe was ionized gas clouds. It only made sense, given that some were ten times the size of the galaxies they inhabited. However, it was quickly discovered that the gas clouds did not have anywhere near the requisite gravity dark matter provided. Black holes were thought of as another possibility, after all, they have so much gravity that they absorb even light. But even black holes cannot provide the amount of gravity needed to keep galaxies together like dark matter does. It would take a million black holes’ worth of gravity to replace the dark matter in an average galaxy! Neutrinos were thought of as the perfect candidate for hot dark matter because they only interact through the weak nuclear force and have an almost-zero mass. However, as discovered by the Sudbury Neutrino Observatory, neutrinos definitely have mass, but not enough to provide galaxies with the gravity needed to stay together. At this point, nothing within the standard model can account for dark matter, so scientists are now considering looking at the supersymmetric extensions of standard model particles. Possible dark matter particles are now classified as WIMPs, or weakly interacting massive particles. Out of the WIMPs, the most promising as of now seems to be the LSP, or lightest supersymmetric particle, because it is stable in almost every supersymmetric theory and only interacts with the weak nuclear force. Now that a particle that fulfills every requisite for dark matter has been identified, it is only a matter of time before it is synthesized and its existence is proven.

Sources:

http://home.web.cern.ch/
The 4-Percent Universe by Richard Panek
"Dark Matter Detection" by P.F. Smith & J.D. Lewin (Physics Reports)

- Jacob Lee