Nearly 50 years ago it was discovered that the fundamental particles, such as the electron and proton, have `left-handed' interactions - they do not respect mirror symmetry. This experimental fact motivates the idea that a set of `mirror particles' exist. The left-handedness of the ordinary particles can then be balanced by the right-handedness of the mirror particles. In this way mirror reflection symmetry can exist but requires something profoundly new. It requires the existence of a completely new form of matter called `mirror matter'. Remarkably the mirror matter theory is capable of simply explaining a large number of contemporary puzzles in astrophysics and particle physics. The evidence ranges from observations suggesting that most of the matter in the Universe is invisible, to unexpected properties of ghostly particles called `neutrinos'. This book explains this fascinating theory and its evidence at a level accessible to the non-specialist.