Astronomers may have obtained a clue about one of the deepest mysteries of the universe - what is it made of?

It is a fact that the galaxies of stars, gas clouds and planets that astronomers can see in space comprise only about 5% of the mass of the universe.

The rest, detected by its gravitational effects, is a mystery. It could be in the form of small sub-atomic particles, dead stars or even super massive black holes.

Macho matter

Astronomers have just boosted the dead star theory with photographs of strange moving objects in space that could be much sought after objects called Machos (massive compact halo objects). These are dark objects whose gravity could explain the motions of our galaxy's visible stars.

Looking at the deepest image ever obtained of the Universe, the Hubble Deep Field, a team of astronomers from the European Southern Observatory, the University of British Columbia and the Hubble Space Telescope Science Institute, have detected five faint objects that appear to be unusual dim stars.

The astronomers compared two images of the Hubble Deep Field taken in December 1995 and 1997 and looked for any objects that moved. Any moving objects would be much closer, inside our own galaxy, than the distant galaxies seen in the deep image.

The stars would be only a few thousand light years from Earth, not over ten thousand million light years like the distant galaxies. It is believed that the stars are Machos moving around our galaxy in the form of a halo of old stars. These would explain the motion of the bright stars in our galaxy

Faded glory

The Macho's are probably old, dim white dwarf stars that are almost as old as the Universe and much dimmer than during their heyday 12 billion years ago.

Although the astronomers have only found five Machos they speculate that there could be several trillion orbiting our galaxy. However, the five possible Machos are not by any means the end of the puzzle of the Universe's unseen mass.

Some astronomers say that if the Universe was full of such stars it would have a higher abundance of heavy elements that it appears to have.

They suggest that while Macho's may swarm around galaxies, other objects fill the space in-between the galaxies and make up a sizeable part of the missing mass. These objects could be small sub-atomic particles, called Wimps (Weakly Interacting Massive Particles).

So, the battle for the mass of the Universe is on, Machos versus Wimps.