Final answer:
Dark energy is an unknown form of energy causing the universe's accelerated expansion, and while theories like the cosmological constant offer explanations based on quantum mechanics, the exact nature of dark energy is still a mystery.
Step-by-step explanation:
Dark energy is d. an unknown nature as yet, and it is the energy that is causing the expansion of the universe to accelerate. Its existence is inferred from astronomical observations, such as the study of distant supernovae and the unexpected galactic redshifts of distant galaxies, indicating an accelerating expansion of the universe. It is distinct from dark matter, which exerts gravity but does not interact with light.
One hypothesis for the source of dark energy is the cosmological constant, an energy associated with the vacuum of space, a concept stemming from quantum mechanics. Yet, attempts to quantify vacuum energy have so far not matched the observed effects needed to explain the accelerated expansion, with theoretical and observational values differing immensely. While quantum mechanics suggests that tiny, ephemeral elementary particles may play a role, the precise nature of dark energy remains one of the most compelling puzzles in modern physics. Despite various theories, the definite source of dark energy continues to elude scientists.