PREFACE My purpose in writing this book is two-fold. First, many non-specialists ask me to explain the mirror matter idea and the scientific evidence for it. Second, science is so specialized these days that many people who know a lot about one field often know little about another. Mirror matter, if it exists, would lead to rather important implications for several scientific fields, including: particle physics, astrophysics, cosmology, meteoritics and planetary science. Thus, it seemed to me that an interesting challenge would be to write a book explaining the motivation for mirror matter and its evidence which could usefully serve these two communities (that is, both specialists and non-specialists alike). Such a venture, though, is not without risks of various kinds. Let me state at the outset that the mirror matter idea is not established fact; it is an example of cutting-edge science in progress. It is my hope that people who read this book will be infected by, or at least understand, my enthusiasm for this subject, and why I think it is one of the most interesting questions in science at the moment. The process of writing this book gave me the opportunity to rethink many of the original arguments. Some `gaps' in my knowledge were filled in, and a few new directions explored. Some material is therefore completely new, although most of it has appeared in the technical scientific literature previously. I have only cited this scientific literature sparingly, but nevertheless I have endeavoured to properly credit the people responsible for the main original ideas. It seems only yesterday that I learned as a student that mirror reflection symmetry was not respected by the fundamental interactions of nature. Electrons and other elementary particles are, in a sense, `left-handed'. Although most scientists have simply come to accept that God is `left-handed', somehow it always bothered me.... One sunny afternoon in May 1991 a rather remarkable thought occurred to me. While playing with an unrelated idea, it suddenly struck me that there was a subtle yet simple way in which mirror reflection symmetry could still exist. Nature's mirror could be unbroken if each type of ordinary particle has a shadowy mirror partner. The left-handedness of the ordinary particles could then be balanced by the right-handedness of the mirror particles. So there you have it, mirror reflection symmetry can exist but requires something profoundly new. It requires the existence of a completely new form of matter called `mirror matter'. At first, it seemed too fantastic to really exist. Yet, over the last few years it appears that almost {\it every} astrophysical and experimental prediction of the mirror matter theory has actually been observed by observations and experiments: There is fascinating evidence for mirror matter in the Universe from astronomical observations suggesting that most of our galaxy is composed of exotic dark material called `dark matter'. Recent particle physics experiments have revealed unexpected properties of ghostly particles called `neutrinos' and weird matter anti-matter atoms. This unexpected behaviour is expected if mirror matter exists. Most remarkable of all is the evidence that our planet is frequently bombarded by mirror matter asteroid or comet sized objects, causing puzzling events such as the huge 1908 Siberian explosion which felled more than two thousand square kilometres of native forests without leaving a single meteorite fragment behind! Altogether I will discuss seven major puzzles in astrophysics and particle physics each arguing in favour of the mirror matter hypothesis. There are indeed seven wonders of the mirror world... New data from current and future experiments will keep coming in even as this book is being printed. Unfortunately, I am not a fortune teller and do not know what these future experiments and observations will find. However, I can predict what they will find if mirror reflection symmetry and hence mirror matter exists. The case for mirror matter will therefore either strengthen or weaken as new data comes in and future experiments are done. In the meantime, I advise you to sit back, relax and let me take you on a journey exploring one of the boldest scientific ideas ever proposed. No scientist works in isolation and I am no exception. I have had fruitful collaborations on mirror matter with a number of very creative people, including Sergei Gninenko, Sasha Ignatiev, Henry Lew, Zurab Silagadze, Ray Volkas and T. L. Yoon. I have enjoyed interesting correspondence on some aspects of this subject with Sergei Blinnikov, Zdenek Ceplecha and Andrei Ol'khovatov. In addition, I would like to acknowledge invaluable support over the years from many friends and colleagues including in particular, Pasquale Di Bari, John Eastman, Greg Filewood, Dave Howland, Girish Joshi, Matthew Tully, and Nick Whitelegg. I am also greatful to many of the above people, and also Jaci Anderson and Glen Deen for providing me with useful comments on the manuscript and Tony Nguyen for helping with the cover. Of course, I thank my family most of all. It is to them that I dedicate this book. Robert Foot