What is dark energy?

a. energy inside a black hole

b. a form of the cosmic microwave background radiation

c. electromagnetic radiation that is not in the visible spectrum

d. whatever is causing the accelerating expansion of the universe

Dark energy is a theoretical repulsive energy that causes the acceleration to the expansion of the universe. Among the choices, the nearest answer would be D. Dark energy can also be defined as a new form of energy, that is a dynamic field that fills up space but has an effect opposite to that of normal energy.