Energy is one of the most fundamental physical parameters. It can be measured, transferred, transformed, and conserved with extraordinary precision. Nevertheless, these properties do not by themselves answer a more fundamental question: What is energy physically?World—Universe Cosmology (WUC) proposes an answer based on the four-spatial-dimensional structure of the World. In WUC, the World is a finite, boundless three-dimensional Hypersphere that forms the surface of an expanding four-dimensional Nucleus embedded in the Eternal Universe. The Nucleus expands in the fourth spatial dimension with the fundamental speed c .A particle in this structure is characterized by four-spatial-momentumP_4=(p_x,p_y,p_z,p_4) where the first three components belong to the directly observable spatial dimensions and p_4 is directed along the fourth spatial dimension. WUC identifies rest energy E as the three-dimensional representation of this fourth component:▭(E=cp_4 )The central proposition of this paper is:Energy is our three-dimensional description of the fourth spatial momentum component, transformed by the fundamental expansion speed c .In this interpretation, four-spatial-momentum is fundamental, while rest energy is an individual-particle property representing its fourth spatial component. The fundamental WUC energy scaleE_0=hcu2044acorresponds to the fundamental fourth spatial momentum scale hu2044a . The purpose of the present paper is to develop this physical interpretation of energy and reconsider the question of where energy comes from.