Recall that the revolutionary reform of the classical physics done by Einstein’s theory of special relativity was brought about by the recognition that the structure of physical space-time is tightly bound with the dynamics of electromagnetic waves. Prior to the revolution, it was tacitly understood that space and time are not physical entities but mathematical ones with which we can describe the structure and the time evolution of a given physical system under consideration. As we have already explained in subchapter 5.1, the core concept in our new off-shell science is spacelike momentum field \(p^{\mu }\) which plays a key role in nonlinear quantum field interactions. Intuitively speaking, space-time can be considered as a field in which quantum field interactions occur, so that it would be reasonable to assume that \(p^{\mu }\) field plays the role of spacelike part of physical space-time. In this chapter, we can show that it is actually the case. After showing this, we are going to explain the unique characteristics of de Sitter space closely related to dark energy field.