The book entitled "[] Space Theory" by Khalid Jerrari offers an exploration of concepts relating to the expansion of the Universe and the rotation speed of spiral galaxies, without using energy or dark matter. Structured in several parts, it begins with a reminder of the foundations of space theory presented in a previous work entitled "The mysteries of space". This concept is based on the notion of "spatial density", a new interpretation of space as a form of energy linked to matter. The book is divided into three parts. The first summarizes the basic principles of space theory, notably the idea that space and matter are inseparable. It also discusses the "principle of universal balance", which suggests that the Universe constantly seeks to maintain a state of dynamic equilibrium. The second part studies the expansion of the Universe through the evolution of spatial density, an alternative to current explanations involving dark energy. The author develops several mathematical models to describe spatial density, proposing functions such as the inverse power law or the increasing exponential function. These models make it possible to simulate universal acceleration and the distribution of mass in the Universe. Finally, the third part examines the rotation curves of galaxies. Based on spatial density, the author explains how the distribution of visible matter can influence the rotation speed of galaxies without resorting to dark matter. The work is intended to be an introduction to an alternative theory of the expansion of the Universe and the rotation curves of galaxies, offering avenues for future research.