Final answer:
The acceleration of the universe's expansion is caused by dark energy, a mysterious force that is not yet fully understood but is inferred from observations of distant supernovae and the increasing rate of the universe's expansion.
Step-by-step explanation:
The question at hand inquires about what might be causing the universe to accelerate its expansion. The correct answer to this question is that we don't fully understand the mechanism yet, but the phenomenon is attributed to dark energy. Unlike MACHOs or white-dwarf supernovae, which are types of astronomical objects, or the hypothetical concept of dark gravity, dark energy is the energy inferred from astrophysical observations that is causing the universe's expansion to accelerate. Observations of distant supernovae have shown that the light reaching us from these exploding stars is fainter than expected if the universe's expansion were decelerating or constant, indicating an acceleration in the universe's expansion. The presence of dark energy suggests that this mysterious energy is pushing spacetime apart, making galaxies and larger cosmic structures move away from each other more quickly as time goes on.