Figure 1 graphically illustrates the concept of the expansion being an energy balance. An artist I’m not, so you have to use your imagination a little. Picture the cube shown in the top repeating to infinity in all directions, like one of those fantastic Escher drawings.

What happens from top to bottom is that matter clumps together locally, radiating energy in the process. In "the real universe," these clumps are visible as stars, star clusters, galaxies, black holes, etc. Because the grid repeats to infinity, there is radiation coming from everywhere, going to everywhere. The result is a uniform glow in all directions. As much radiant energy enters each cube as leaves it.

Thus, the total energy content of the universe, from top to bottom, never changes. What changes is the way this energy is distributed. In the top, gravitational energy is uniformly distributed among all particles. In the bottom, energy has been “lost” by matter within each clump. Yet with respect to other clumps, the matter within each clump has “gained energy” by moving apart.

Keep in mind, it is not just the cube shown that has expanded; every cube all the way to infinity has done likewise. So the distance between all clumps has increased, and the greater the starting distance, the greater the increase in ending distance, i.e. Hubble’s law. The time-scale from top to bottom is about a billion years. Also, it is the bizarre tenets GR that allow space to stretch, as depicted (and observed), but it is the more mundane principal of Conservation of Energy that drives the expansion.