You, the computer you’re sitting at, the air you breathe, even the distant stars are all made up of protons electrons and neutrons. For a long time this ordinary matter, or what physicists like to call baryonic matter, was thought to be the main constitute of the universe. However, in the past twenty years evidence has been accumulating to the contrary, that in fact the universe is much stranger than ever thought of before and is almost entirely made up of something that we can’t see.

For along time astronomers weren’t concerned about the mass of objects that they couldn’t see. For example the earth is too small and dim to see from any great distance and all the planets in out solar system make up less than one percent of the total mass of the sun. However it soon became a concern when astronomers began to measure the mass of galactic clusters and it became apparent that there was a significant amount of matter unaccounted for.

In the thirties, astronomers named Zwicky and Smith both examined closely two relatively nearby clusters, the Coma cluster and the Virgo cluster. They looked at the individual galaxies making up the clusters individually, and the velocities of the clusters. What they found was that the velocities of the galaxies were about a factor of ten to one hundred larger than they expected.

 

Virgo Cluster		Coma Cluster

 

In a cluster the main force is the gravitational pull of the galaxies on one another which gives rise to their velocities. By knowing the velocities of the galaxies the total mass of the cluster can be determined.

If your web browser is Java-aware -- e.g., Netscape 2.0b or higher, try this experiment. It allows you to vary the mass inside a galaxy cluster, and watch the individual galaxies.

Experiment I

( Courtesy of John's Homepage http://www.astro.queensu.ca/~dursi/dm-tutorial/dm1.html)

Now like all observations there is a certain amount of error involved. In this case, watching the galaxies in a cluster takes years of observation and the velocities are hard to determine due to the expanse of the cluster. It’s not like the experiment were the dots are whizzing around. Also some of the galaxies measured may not be in the cluster but are just in the line of site of the telescope. So even as strong as this evidence seemed to appear it was mostly ignored due to the errors of the observation.

 

 

Forty years later people like, Rubin, Freeman, Peebles, started to measure the rotation curves of galaxies to determine their mass. Masses such as galaxies spin around their center and follow Kepler’s Laws for orbits about a center axis. These laws state that the rotational velocity around the center depends only on the distance to the center, and the total mass that is contained within the orbit.

Here is another experiment. This one allows you to vary the mass inside an orbit, which changes the rotation velocities.

Experiment II

( Again courtesy of John's Homepage http://www.astro.queensu.ca/~dursi/dm-tutorial/dm2.html)

 

 

The mass discrepancy found through observing the velocities and rotations of galaxies has been confirmed through gravitational lensing, the bending of light predicted by Einstein's theory of general relativity.

 

	By measuring how the background galaxies are distorted by the foreground cluster, astronomers can measure the mass in the cluster. The mass in the cluster is more than five times larger than the inferred mass in visible stars, gas and dust.
Courtesy of Nasa WMAP mission