Why does dark matter matter?

Ordinary matter are the electrons, protons and neutrons which from the atomic elements like Carbon, Oxygen and Uranium, which in turn form Earth, the Sun and Human beings. This type of matter is able to interact with electromagnetic radiation (essentially ‘light’), is visible by things like telescopes, and is generally well understood.

Cosmologists (people who study the evolution of the Universe) have a pretty good idea of how much matter is in the Universe, but observations of rotating galaxies and the structure of the Universe suggest that there must be something else apart from ordinary matter.

In fact, the observations tell us that ordinary matter makes up only 5% of the mass of the Universe! The other 95% is thought to be made up of dark matter and dark energy. Various dark matter candidates include very fast moving particles such as neutrinos, stuff similar to ordinary matter called MACHOS (massive compact halo objects), and most probably WIMPS (weakly interacting massive particles). All this dark matter accounts for around 25% of the ‘missing mass.’ The rest of the universe is made up of something even stranger called dark energy. While it may sound as though it belongs in a Sci-Fi film, it as an attempt to reconcile the expansion of the Universe with the observable energy in the universe. Dark energy can be thought of as a ‘negative’ energy which is ‘pushing’ the universe outwards – although I use these terms very loosely. Both dark matter and dark energy are almost impossible to detect directly because they do not behave like ordinary matter. Instead, scientists theorize based on the indirect effects seen throughout the Universe. While it seems certain that ordinary matter is a very small part of the Universe, it will be sometime before the rest is fully understood.

This article appears in Melbourne University's Student Newspaper Farrago, Edition 2, 2006