Since Newton's time, Astronomers have delighted in the powerful method of finding new objects in space through the effects of their gravity, before the objects themselves are actually seen directly. The first great success of this idea was the discovery o f Neptune, by the reconstruction of where it must be in the Solar Sytem in order to gravitationally perturb the motions of the other planets. Similarly the inference that some of the bright nearby stars must be circled by faint but heavy companions lead to the discovery of white dwarf stars.

This venerable technique is still in use today, as it is currently being used to infer the existance of large amounts of unseen matter around galaxies, in clusters of galaxies , and perhaps even dominating the Cosmos as a whole. The mystery of the dark matter began in the 1930's, when it was noticed that the motions of individual galaxies in large clusters of galaxies was so rapid that the clusters ought simply to fly apart.

The matter deepened in the 1970's and 1980's when it was discovered that the motions of the stars within individual galaxies were so rapid that they ought to fly apart, too. If our understanding of Newtonian gravity is correct, then the implication is that there is a lot of matter in these structures, holding them together, but which we cannot yet see.
 

The most conventional idea for what these dark objects could be is in faint red stars, stars which are so intrinsically faint that they could not be detected directly with our best telescopes until the launching of the Hubble Space Telescope . Some such stars are known to exist, since a few happen to be close to the Earth, but if th e dark matter in the Galaxy were composed of such stars, the vast majority would be so far from us that ground based telescopes are unable to distinguish between these faint stars and the much more numerous faint galaxies. These stars are red because they are smaller than the sun and their surfaces are cooler.  The superb resolving power of Space Telescope allows us to get around this problem for the first time: with the ten times sharper images we get from space, above the distortion of Earth's atmosphere, the difference between faint stars and faint galaxies can be easily seen.
 

At Tuorla Observatory, we have examined the images taken of the so-called Hubble Deep Field, and saw far fewer faint stars than predicted if the dark matter were made entirely of these objects. The Hubble Deep Field is a one week long exposure taken with the Space Telescope of a small patch of sky; it shows the faintest stars and galaxies ever seen.
 

Case dismissed!

If the dark matter in our own Galaxy were composed of faint red stars, then we would expect to see dozens or even hundreds of them on this image, whereas we found no red stars at all. This puts a strict limit on the amount of dark matter which could be in faint objects of this type.

This leaves the possibility that the dark matter is made of something fainter still, such as brown dwarfs, (basically big versions of Jupiter) which are not hot enough internally to begin nuclear fusion and so do not glow like true stars. Some of these long predicted objects have been actually located in just the last two years, but they would be too faint to see in this image. Then again, the dark matter may be something completely different, such as fundamental particles like the neutrino, or WIMPS, or axions or black holes (quite a list!); or we may even need to modify the laws of Gravity. All of these possibilities and more are in active research around the world.

A faint cluster of galaxies observed with Hubble Space Telescope. Clusters contain anything from a few to a few thousand galaxies bunched together and moving around within their combined gravitational field. The Hubble image above shows distorted images of background galaxies which have been "lensed" by the dark matter which is holding this cluster of galaxies together.

" Rotation curve " of the Milky Way Galaxy showing how fast stars move in circular orbits as a function of their distance from the center of the galaxy. One expects the stars to get slower the further they are from the center, whereas their speed remains instead close to constant. This indicates that there is matter in the Galaxy whose gravity controls the stars almost totally. And yet we cannot find this matter!

The NASA/ESA Hubble Space Telescope during the mission to upgrade instruments (STS-61). Astronauts Story Musgrave and Jeffrey Hoffman are seen during the last of the five EVAs. Australia's west coast can be seen in the background.

The faintest stars and galaxies ever seen. There are thousands of galaxies and just a handful of stars (shown inside yellow circles) on this image. The number of stars is so small that they can in no way account for the large amount of "dark matter" thought to dominate the mass of the Milky Way Galaxy.