Dark Matter :: physics science space

Missing Diagrams 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 a long 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. 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.