Astronomers have mapped of one of the most massive structures in the Universe, showing how much more there is to it than glowing stars and gas.
By Dr David Whitehouse
BBC News Online science editor
The object is a distant cluster of galaxies that contains dark matter, the unknown component that comprises most of the mass of the Universe.
Dark matter is shaded blue
The new "mass map" shows it is distributed in much the same way as are the visible stars.
Researchers believe it will lead to a better understanding of the mysterious dark matter, even though it does not help them find out exactly what it is.
Here, there and everywhere
Clusters of galaxies are the largest stable systems in the Universe. They contain not only stars, gas and dust but something else as well.
Astronomers do not know what that something is. They know it is there because they can see the effect its gravity has on the motions of the stars and galaxies.
Dark matter makes up about 80-85% of the matter in the Universe. Many types of objects are suspected, ranging from dead stars to hoards of sub-atomic particles.
In an attempt to pin down the distribution of dark matter, whatever it is, an international team of astronomers used the Hubble Space Telescope to make its mass map of the galaxy cluster called CL0024+1654.
The map enabled them to see how the dark matter is distributed with respect to the visible galaxies in the cluster.
Across the Universe
To produce the map, the researchers looked at more distant galaxies behind the cluster, noting that their shapes are distorted by the gravity of the foreground cluster - an effect called gravitational lensing.
...and just the stars
The nature of the distortion provides information about the unseen mass of the cluster.
The project required more than 120 hours observing time - the largest amount of Hubble time ever devoted to studying a galaxy cluster.
The investigation has resulted in the most comprehensive study of the distribution of dark matter in a galaxy cluster so far.
The map reveals that the dark matter drops sharply with distance from the cluster centre, which is what astronomers expected.
"This confirms a picture that has emerged from recent detailed computer simulations," says Professor Richard Ellis of CalTech, US.
Ellis and colleagues presented their study at the General Assembly of the International Astronomical Union. They will also publish their results in a forthcoming issue of Astrophysical Journal.