Final answer:
The acceleration of the universe's expansion is caused by a mysterious form of energy known as dark energy, which opposes gravity and causes spacetime to expand at an increasing rate.
Step-by-step explanation:
The phenomenon responsible for the acceleration of the universe's expansion is known as dark energy. Much like stepping on the gas pedal provides energy to accelerate a car, some form of energy is necessary to accelerate the expansion of the universe. Despite not being fully understood, dark energy was proposed after scientists observed distant supernovae and noted the universe's expansion to be speeding up. Unlike dark matter, which exerts gravity and contributes significantly to the universe's total mass, dark energy is theorized to work against gravity, pushing spacetime apart, and facilitating the acceleration of the universe's expansion.
Dark energy becomes increasingly dominant over gravity as the universe ages, which suggests that its effects are becoming more pronounced leading to an accelerating expansion. The concept of WIMPs or weakly interacting massive particles, pertains to dark matter rather than dark energy and is not responsible for the universe's accelerated expansion. Therefore, the correct answer to what might be causing the universe to accelerate is E) We don't know!—but we call it "dark energy".