The ‘four percent’ in the title of this book refers to the apparently true but bizarre fact that only 4% of the universe seems to be ordinary stuff – from planets to stars – with twenty-odd percent of the remainder dark matter and the rest dark energy, the unknown phenomenon that is forcing the expansion of the universe to accelerate. Don’t come to this book hoping to find out what dark matter and dark energy are – because there’s a long way to go before those questions can be definitively answered – but instead you will find an in-depth history of the process by which the (probable) existence of dark matter and dark energy were discovered. Richard Panek is at his best when describing human beings in action, rather than covering the details of physics or cosmology. He really takes the reader in to experience the astronomers, astrophysicists and cosmologists (surprisingly different beasts) at work. We begin to understand how these people work, what drives them and what they rea...