Dark matter: Material answers

Abell was I

THE 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.

(more of the article)