Andrew Francis, of Waimate High School, asks :-

What is dark matter, where is it found and how do we know it is there?

Phillip Yock, a physicist at the University of Auckland and who is engaged in the search for dark matter, responded.

Dark matter is a hypothetical form of matter that is conjectured to exist in inter-stellar and inter-galactic space. Astronomers have estimated that at least 90 per cent of all matter in the Universe is dark, but have until recently only speculated about its actual composition. Experiments that are under way now may determine its composition.

Dark matter is believed to be present in spiral galaxies because the stars orbiting around the centres of these galaxies are moving too fast to be bound by the gravitational field of the matter we can see in these galaxies. The stars would be 'flung off' into inter-galactic space if it were not for the extra gravitational pull of unseen matter that must be present. Most galaxies, including spiral galaxies, are members of clusters of galaxies, and these exhibit a similar effect.

The above arguments are regarded as nearly compelling. They assume only that the laws of gravity as deduced by Newton for planetary motion about the Sun apply equally well over galactic distances. The Big Bang theory also requires dark matter. The remnant radiation from the Big Bang that is observed today is almost uniform in all directions, yet the matter we see in the form of galaxies and clusters of galaxies is far from uniform. Gravitational attraction alone could not have caused the clumping unless it was boosted by additional matter and gravity from invisible material.

What could dark matter be? One possibility is that it is comprised of huge numbers of elementary particles called 'neutrinos'. These particles emit no light. They could account for the observations if they have a small mass, about one hundred thousandth of the electron's mass. Another possibility is that numerous stars exist that are invisible, or shine very feebly. These could be 'black holes' or small stars known as 'dwarf stars'. New Zealand is well placed to test the latter possibility. Measurements are being made from the Mt John University Observatory in Canterbury that could uncover black holes of dwarf stars in our galaxy. A technique that utilises Einstein's theory of general relativity is being used for these measurements.