If the expansion of the Universe is accelerating, must there be more energy or a less mass, or both ?

Answer 1

It is generally accepted that the acceleration implies the existence of more energy. It has been given the name "dark energy," inspired in part by the name "dark matter," and for a similar reason. It can only be detected by its effects, and its exact nature is unknown.

Answer 2

The above answer is correct, scientists admit that dark energy was proposed to explain the expansion but if it does exist they have no idea of where it came from.