Space Is Not Empty — It's Full of Invisible Structure

What we call "empty space" is pervaded by quantum fields, dark energy, and the cosmic microwave background — the afterglow of the Big Bang. Even a perfect vacuum contains virtual particles constantly blinking in and out of existence due to quantum uncertainty. Dark energy, which makes up ~68% of the universe, acts throughout all of space, driving the universe's accelerating expansion.

Source: The Fabric of the Cosmos — Brian Greene