Galaxies do not usually exist alone. They tend to bunch together in small groups, like the Local Group of galaxies in which the Milky Way sits, or larger groups called clusters. This is useful for cosmologists, as it gives them a chance to study one of the most elusive substances in the universe: dark matter.

Galaxy cluster MACS 1206 as seen by the Hubble Space Telescope (click for a bigger version). Credit: {link url=""}NASA, ESA, M. Postman (STScI), and the CLASH Team{/link}

Dark matter makes up 23% of the universe but we know very little about it. A multi-wavelength survey called the Cluster Lensing And Supernova survey with Hubble (words which together make the obviously completely unintentional acronym CLASH) hopes to change that, by observing 25 clusters of galaxies in greater detail than ever before. The CLASH team have already completed observations of six clusters and plan to finish another five before the year is out.

MACS 1206 is a galaxy cluster that was recently surveyed by CLASH. It lies 4.5 billion light-years from Earth in the constellation Virgo in our night sky. The image of MACS 1206, above, was taken between April and July 2011 with Hubble's Advanced Camera for Surveys and the Wide Field Camera 3. The observations, along with observations of other galaxy clusters to be surveyed by CLASH, will help astronomers construct detailed maps of dark matter in galaxy clusters.

Galaxy clusters are the perfect test site for dark matter's gravitational effects because they are the biggest gravitationally bound objects in the universe. There are three defining features of a cluster: they contain hundreds of galaxies (at least — sometimes thousands); between the galaxies there are huge clouds of hot gas (we're talking a hundred million degrees); finally, they contain dark matter, and lots of it.

We can't see this dark matter — that's where the 'dark' bit of its name comes from. We can, however, work out how much of it a galaxy cluster harbours using an effect known as gravitational lensing. Because of their huge masses, galaxy clusters act like giant 'gravitational lenses', bending and magnifying light that passes through them. This includes light from galaxies that lie beyond the cluster in our line of sight.

Gravitational lensing can cause the same galaxy to show up twice or more in different parts of an image, and can change how that galaxy looks in the image too.

The amount a galaxy is distorted depends on the total amount of mass in the cluster. The total mass includes that from the galaxies, the gas between the galaxies, and the dark matter. Astronomers are able to use the distortion of galaxies that lie behind galaxy clusters in our line of sight to measure the mass of the intervening galaxy cluster.

They can measure the amount of 'normal' matter in a galaxy cluster, so they know how much mass a cluster should contain if there were no dark matter.

Then, by using the amount of distortion to measure the total mass of the galaxy cluster, astronomers can work out how much dark matter exists within a particular cluster and how that dark matter is distributed. The distribution of dark matter in a cluster can give them some clues about how and when that galaxy cluster formed.

When Hubble surveyed MACS 1206 it found 47 multiple images of 12 galaxies that lie behind the cluster. A lot of the galaxies in the Hubble image of MACS 1206 look like they have been smeared slightly. If you look closely, you'll notice that they are mostly smeared so that their longest edge is facing the centre of the image, making it look like they are in some kind of whirlpool. They are facing the centre of the galaxy cluster. By measuring how much the galaxies are smeared out, and in what way, astronomers hope to gain one more piece in the puzzle that is dark matter.