Dark matter is out there, but you can't see it
Astronomers have detected a "cosmic web" in the most accurate image yet obtained of the dark matter in a supercluster of galaxies.
Even though the exact nature of this invisible material remains unknown, astronomers are able to produce detailed maps of the cosmos, showing its location in relation to the ordinary matter that we can see in telescopes.
Dr Andrew Taylor of the Royal Observatory, Edinburgh, UK, says: "Ultimately we hope that the detection of a dark matter particle in a laboratory will reveal its true nature and place in physics.
"But at the moment our only guide to the properties of the dark matter is from cosmology."
Cosmic web
The method used by Dr Taylor and Dr Meghan Gray is to take advantage of gravitational lensing - when light from a distant galaxy is bent by the gravity of matter in front of it. Such gravitational lenses provide a direct probe of where dark matter is in the Universe.
The researchers have made the most detailed analysis yet of the dark matter in the Abell 901/2 supercluster, one of the largest known structures in the Universe.
This enormous structure, about 10 million light-years across, contains a group of galaxy clusters known as Abell 901a, 901b and Abell 902.
The team's image of the dark matter in the system, which covers an apparent area of sky the size of the full Moon, was obtained by analysing the gravitationally lensed images of 50,000 galaxies.
It shows not only that the galaxies we see lie within larger dark matter clumps, but that these clumps are connected by "cosmic filaments" - bridges of dark matter that connect the clusters.
The existence of these filaments has long been a prediction of the theory of dark matter - that an intricate network of clumps and filaments, the so-called cosmic web, pervades the Universe.
More recently, Dr Taylor has developed a new method which will allow cosmologists for the first time to make fully three-dimensional images of the dark matter distribution in the Universe.
"Having 3-D images will allow cosmologists to put much better constraints on the nature of dark matter in the Universe," he explained.
The results have been published in the Astrophysical Journal and presented at the UK National Astronomy Meeting in Bristol.