In the past, the now-well known conclusion that the universe is constantly expanding due to the force of dark energy was not so obvious. The universe was for a long time thought to be in a process of constant shrinking due to the force of gravity, and more recently was believed to be in a temporary and decelerating rate of expansion.
The dark energy theory as we know it today has actually been a recent creation. It was not until hte 1990s that measures of the Cosmic Microwave Background conclusively proved that the universe was in a constant state of expansion, and that the expansion was accelerating, although the theory of the expanding universe was far older, dating back to the 1800s.
Today, the theory continues to evolve, and its explanation has integrated hte internal logic of ergodicity, or the nature of things to dissipate until a substance is relatively standard over a space.
The present theory of the expanding universe thereby offers a complete and simple explanation of how the universe expands. The force propelling the expansion seems to be a reverse form of gravity over large spaces ,conforming to Einstein’s space time theory, while the galaxies are becoming constantly scattered among the open spaces due to the tendency of all things towards Ergodicity.
I found it really interesting how much the theories of hte universe had shifted even in the last 20 years – just one generation ago, we in astronomy classes might have been taught about the big crunch, not about dark energy!