I spend a lot of time on this blog explaining a physical spacetime model and the underlying metaphysics. In this series of posts, each entry poses a physics question for the spacetime model, along with the answer.
Physics Qustion #4. The Cosmological Constant Problem (Dark Energy Problem).
Measurements of the cosmic microwave background radiation confirm that the universe is flat. There is not enough matter in it to explain this, so it is thought that there must be some "dark energy" that acts like Einstein's cosmological constant and has just the right value to make the universe flat. Supernova measurements show that the universal expansion is accelerating, and the dark energy is thought to be responsible for this effect as well. Physicists have no idea what this dark energy might be or how it gets so finely tuned as to make the universe perfectly flat. The best candidate, vacuum energy density, doesn't work, because the cosmological constant is 120 orders of magnitude smaller than the vacuum energy density calculated using the Standard Model.