What is Dark Matter?

Dark matter, the term, as used by astronomers, means a form of mass which does not interact with electromagnetic radiation, and which is “cold” (which means it’s not zipping along at some big fraction of the speed of light). As the only forms of dark matter that have been discovered here on Earth – neutrinos – are not cold, and as the Standard Model of particle physics is so successful, cold dark matter is thought to be non-baryonic (i.e. not composed of baryons). So “dark matter” means “cold, non-baryonic dark matter”.

The history of dark matter is fascinating, not least because the meaning changed a lot from its first use, by Zwicky in the 1930s, to today. Zwicky found that the galaxies in some clusters he studied seemed to be moving far faster than they should if the only matter in the clusters was the stars, gas, and dust in those galaxies. The most obvious answer was that the galaxies contained far more mass (matter) than in those galaxies; he called this unseen mass ‘dark matter’ … and he was right. Many decades later, x-ray astronomers discovered that most of the (baryonic) mass in galaxy clusters is in a very hot plasma between the galaxies, not matter in the galaxies. Yet still the total mass of these rich clusters is less than the mass of the visible (in x-rays!) plasma (plus galaxies, etc).

Many decades after Zwicky’s discovery, Rubin and Ford discovered that the stars, gas, and dust in the outer parts of spiral galaxies were moving faster than they should, if the only mass in those galaxies was the visible stars, gas, and dust.

At the scale of the universe as a whole, several sets of detailed astronomical observations (e.g. those of the Cosmic Microwave Background) also point to the existence of a form of matter which interacts with other matter only via its gravity (and, possibly, by the weak nuclear force).

Here’s the most astonishing thing: the amounts of dark matter, from all kinds of astronomical observations, are the same; for every kilogram of ordinary matter, the universe contains approximately six kilograms of dark matter.

But what is dark matter made up of? No one knows.

Would you like to read more? Here are some good resources: Dark Matter – Introduction (NASA), Dark Matter (from an online astronomy course), and Dark Matter (University of Washington).

Universe Today stories on the question of what is dark matter? Lots, and I mean LOTS! Here is a small sample: ESA’s Tough Choice: Dark Matter, Sun Close Flyby, Exoplanets (Pick Two), What Can the (Dark) Matter Be?, and Galaxy Collision Separates Out the Dark Matter.

Astronomy Cast episodes of relevance to the question what is dark matter include the March 31st, 2009 Questions Show, the September 28th, 2008 Questions Show, and Gravitational Lensing.

Tagged as: Cosmology, Rubin, Zwicky