A shadow universe

A bizarre universe may be lurking in the shadows

Such hidden worlds might sound strange, but they emerge naturally from complex theories such as string theory, which attempts to mesh together the very small and the very large. Hidden worlds may, literally, be all around us. They could, in theory, be populated by a rich menagerie of particles and have their own forces. Yet we would be unaware of their existence because the particles interact extremely weakly with the familiar matter of our universe. Of late, physicists have been taking seriously the idea that particles from such hidden sectors could be dark matter.