Most of the matter in the Universe is not the ordinary kind made up of protons, neutrons, and electrons, but an elusive "dark matter" detectable only from its gravity. Dark matter is all around us --- it goes through us all the time without us noticing --- but tends to collect in large quantities around galaxies.

New research by Ignacio Ferreras (University College London), Prasenjit Saha (Queen Mary, University of London), and Liliya Williams (University of Minnesota) has mapped the mass distribution in 18 galaxies up to ten billion light-years from us. Dr Ferreras and his colleagues found a surprising trend between the proportion of dark matter and the mass of the galaxy, so that the biggest galaxies are dominated by dark matter.

Dr Ferreras and his colleagues took advantage of a rare astronomical phenomenon known as 'gravitational lensing'. The 18 galaxies they studied serendipitously lie in front of quasars, which are bright sources of light but even further away. The gravity of the galaxy and the dark matter distorts the quasar light, causing each of the quasars to be seen as two or four images. The placement of these mirage images, studied using new theoretical techniques in gravitational lensing, makes it possible to measure the total mass. By analysing the starlight from the galaxies using stellar evolution theory, it is possible to measure the stellar mass. Combining these ideas on archival data from the Hubble Space Telescope, Dr Ferreras and his colleagues were able to make dark-matter maps.

Current theories of galaxy formation can explain some of these new findings but not all. After the Big Bang, gas will tend to fall towards the centres of dark-matter halos and there ignite to form stars and thus a galaxy. But why is the dark-matter fraction higher in more massive galaxies? And is the formation process already complete in these galaxies from ten billion years ago? These are some of the puzzles that need to be explained by a future theory of galaxy formation.

The technical paper by Dr Ferreras and his colleagues appeared in the April 10, 2005 issue of the Astrophysical Journal Letters. A link to the paper can be found here.