Gates starts with a look at our current understanding of the universe based on General Relativity (GR), and goes on to describe how this subject was developed, including Einstein's use of the cosmological constant. Most of the book is then about how gravitational lensing, based on GR, is used to investigate the nature of dark matter. Watching for a blip in the light curve of a star might indicate a massive body, such as a planet or even a black hole, passing in front of it. Further afield, the observation of distortion and multiple copies of a distant object can be used to calculate the mass of a galaxy or cluster in front of it, and show that its much more than is accounted for by visible material. More subtle is Weak Lensing, where distortion is used to build up a statistical picture of the composition of the universe. Gates also describes what is being done on Earth tofind out what dark matter is made of, and the book ends with a look at how dark energy is being investigated in cosmological models.
It's a well written book, being accessible to those without much previous knowledge of the subject, but with sufficient new material to keep the interest of more experienced readers. I'd recommend it to anyone who wants to find out more about what our universe is made of.