ESO has today released a wide-field long-exposure image that reveals a sea of galaxies, including a group belonging to the dark matter-dominated cluster Abell 315.

The new image was taken with the Wide Field Imager on the MPG/ESO 2.2 metre telescope at ESO's La Silla Observatory in Chile and reveals thousands of galaxies packed into an area just the width of the full Moon. Spanning a vast range of distances, those nearest to us can be identified by their spiral arms or elliptical halos, while the furthest are faint pin-pricks of light.

Roughly one hundred yellowish galaxies concentrated around the centre of the image and extending out to the bottom left make up the cluster Abell 315, located two billion light years away towards the constellation Cetus. There is more to this giant cluster than meets the eye. Just ten percent of its mass is actually made up from galaxies, with another ten percent coming from hot gas that persists between them. The 'missing' mass comes from dark matter, an invisible and uncharacterised component of our Universe that reveals itself only through the gravitational influence it exerts on its surrounds.

The huge mass of a galaxy cluster acts like a giant cosmic magnifying glass, bending the trajectory of light from galaxies situated behind it. By studying the shape and extent of the apparent distortions astronomers can infer the total mass of the cluster responsible for the distortion, even though it cannot be seen. In the case of Abell 315 nearly 10,000 faint galaxies were studied to conclude that the total mass of the cluster amounts to over one hundred thousand billion times the mass of our Sun.

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