by Dr. Sten Odenwald

Abell520Fritz Zwicky and Vera Rubin Had no idea at the time that the ‘missing mass’ they had discovered would eventually lead to one of the greatest mysteries facing astronomers in the 21st Century. Zwicky had discovered that galaxies were moving faster within clusters that could be supported by the idea that clusters of galaxies were stable over the span of billions of years. Later Rubin in the 1970s, found that stars in the distant suburbs of spiral galaxies were also moving faster than their expected Keplerian speeds, again suggesting lots of missing mass in many normal-looking galaxies.

The search started for additional kinds of normal matter to explain this missing mass, including very hot plasma (hot intracluster medium) and dwarf stars (faint galactic halos), but despite some interesting leads, nowhere near the right amount of this new ‘baryonic matter’ could be scraped up. What became more of a problem is that, if you proposed that normal matter in some observationally ‘dark’ form was the cause of this missing mass, and you added this to the nuclear chemistry of the Big Bang, you would get a different answer for the ratio of primordial helium to hydrogen. Even if you used black holes, they at one time were normal matter, and would disturb the observed 1:4 ratio of cosmological helium to hydrogen, as well as the abundance of deuterium.

Based on decades of modeling galaxies, clusters and the formation of large-scale structure in the universe, modelers decided that this missing mass could not be ‘hot dark matter’ because it would interfere with the formation of small-scale galaxies in the universe. It would have to be ‘cold dark matter’, which would actually stimulate the formation of galaxies.

Physicists got into the game in the 1970s and proposed that, although baryons would remain what we observe in stars and gas in the universe, it is possible that neutrinos carried a smidgeon of mass, and this ‘non-baryonic’ neutrino mass would be able to explain missing mass. This didn’t pan out in the 1990s when laboratory experiments verified the upper limits to neutrino masses being far less than what would be needed to solve the missing mass problem.

They then worked backwards from the cosmological calculations and came up with the idea that missing mass had to take the form of weakly-interacting particles that carried a lot of mass, hence WIMPS. At the same time that the search for a grand unified theory beyond the Standard Model was being conducted in the 1970s and 1980s, a parallel search for particle candidates for WIMPS also too place, and this is where we are today.

The most fashionable theories that take us beyond the Standard Model incorporate supersymmetry; a new symmetry between fermions (quarks and leptons) and bosons (photons, gluons, Higgs etc), which solves many thorny problems in the search for a better unification model for matter and forces. The particle that keeps falling out as the best candidate for dark matter is called the neutralino, a weakly-interacting particle with a mass between 100 GeV and 10 TeV. This is the ONLY particle type which has all of the necessary creds to be the explanation for dark matter. No other particle in the Standard Model will work, despite decades of trying to force-fit known particles into this cosmological mystery.

So here we are today. All of our cosmological eggs are now in only one basket. Just as the Higgs boson was the be-all, end-all particle to complete the electro-weak model developed 50 years ago, the idea of supersymmetry and the inevitable dark matter neutralino now also has a 40+ year history of inevitability, but still lacking confirmation.

What is worse is that the latest results from the Large Hadron Collider in CERN have sent a chill down the backs of all physicists who want supersymmetry to ‘work’. So far, none of the expected supersymmetry partners to the neutralino, including the neutralino itself, have yet been found at the energy range from 100 GeV to about 5 TeV. If this particle desert continues to expand to 15 TeV when the LHC is restarted in 2015, not only will supersymmetry fall as a hot new idea, but it will also sweep away the only explanation for dark matter that astrophysicists have been able to come up with in 50 years.

The stakes could not be higher, so stay tuned for the resolution to this cosmic mystery in the years to come. By 2020 we will know on which side of the bread our universe has been buttered!


StenPictureDr. Sten Odenwald is an astronomer and educator at the National Institute of Aerospace and NASA/Goddard, who also runs an online resource called The Astronomy Cafe. He received his PhD in astrophysics from Harvard University in 1982. He is an active science popularizer and book author, and has a number of websites promoting science education and mathematics: The Astronomy Café and [email protected] He has also appeared on National Geographic TV specials and a number of YouTube video productions. His latest article 'String Theory' appeared in Astronomy magazine, and 'Preparing for a Solar Superstorm' in Scientific American. His latest e-book is 'Mind, Space and Cosmos: Exploring the mystery of space and how we think about it'. He writes a weekly blog for the Huffington Post at


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