Abell was ITHE Higgs boson (see article) is not the only curious form of matter whose nature has been probed this week. A paper by Jörg Dietrich, of the University of Michigan, and his colleagues, just published by Nature, illuminates—if that is the appropriate word—a substance known as dark matter.
Dark matter, the theory goes, is composed of particles that cannot interact with the electromagnetic force, and thus have no dealings with light. But they do interact gravitationally. In fact, it is the gravitational pull of dark matter that stops galaxies flying apart as they rotate. Moreover, calculations suggest there is five times as much dark matter in the universe as there is ordinary matter. But what is rarely observed is dark matter by itself. Since both the dark and the visible forms of matter are affected by gravity, they tend to cluster together.
Models of the evolution of the universe suggest, though, that this clustering is secondary. The young universe was first filled with a lattice of threads of dark matter, then the visible stuff gathered around these threads and formed the galaxies familiar today.
What Dr Dietrich and his colleagues have done is to detect the part of a thread that runs between two groups of galaxies called Abell 222 and Abell 223. They were able to do so by looking at the distorting effect the thread’s gravity has on light emitted by galaxies behind it. Measuring these distortions allowed the researchers to work out both the thread’s shape and its mass (about 60 trillion times the mass of the sun). Meanwhile, down on Earth, researchers at CERN, the particle-physics laboratory near Geneva that has just found the Higgs, will now turn their attention to making individual particles of dark matter. Thus do the largest and the smallest scales of science complement each other.
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