A few years ago scientist thought they were well on the way to understanding the universe. New tools allowed them to look further and to see more than ever before. One thing that they needed to do is to get some idea of the amount of mass that the universe contained. This should be easy. The matter that we have here on earth is the same kind of matter that shines in the stars. Picking a representative area of the cosmos and getting an inventory of what is visible should give us the answer. Imagine their shock when they found that the normal matter that we all know is only 5% of what the universe contains. A type of matter called dark matter is 23% and the rest is dark energy at 72%. How can we find out about dark matter? We can't see it. As it behaves just like normal matter with respect to gravity, we can find out how much there is by how it influences the normal matter that we can see. It is the motion and speed of the things we can see that tell us it must be there. Dark energy is even harder to pin down. The explanation of why scientist believe it's there is a bit more convoluted, so I'll leave that one up to the author.
One thing that helps us find out about dark matter is Einstein's telescope. We have much better telescopes and other instruments today than we ever had but Einstein's telescope is not one that we can build. Einstein's theory told him that when light from an far object in space passed near an object closer to us like our sun the light would bend so that the apparent position would be shifted from the actual position as observed from the earth. It took an eclipse of the sun with a very bright object slightly behind it and a few years but conditions were finally right and Einstein's prediction was proven. It is the bending of light through a lens that makes a telescopes work. So light bent by a mass in space makes it act like a telescope. This means that, we can use massive objects in space to see images of the objects behind them. The image is somewhat distorted and sometimes multiple images of the same object are created but we can often see objects that we could never see with our normal telescopes. It's best when the massive object is about half way between us and the distance object. Some of the most interesting results come from when the massive object is a cluster of galaxies. But the other important thing is what we learn about the lens. The angle that it bends the light tells us it's mass and every thing that isn't visible normal matter is dark matter. There are other little tricks to getting it right, so again, I'll leave that to the author.
This book takes you on a journey from the universe of Einstein to the universe as it is understood today and beyond to the edge of what is unknown and maybe unknowable. Evalyn Gates is the perfect person to take us on this journey. She is the assistant director of the Kavli Institute for Cosmological Physics and also a senior research associate at the University of Chicago. She is the former astronomy director of the Adler Planetarium and Astronomy Museum. She makes a complex subject accessible to the average reader. She doesn't assume that you have the prior knowledge and background to understand the subject matter. She uses the first three chapters to make sure you have the background to understand the rest of the book. If you want to get the latest knowledge for understanding what is known about our universe, this book, with it's accessible explanations and many helpful illustrations is the one to get. The author tells you to start with the beautiful color pictures of actual telescope images located in the middle of the book, just for their beauty. Then go back to them as they illustrate the topics in the book.