So what is dark energy? Well, the simple answer is that we don't know. It seems to contradict many of our understandings about the way the universe works.
We all know that light waves, also called radiation, carry energy. You feel that energy the moment you step outside on a hot summer day.
Einstein's famous equation, E = mc2, teaches us that matter and energy are interchangeable, merely different forms of the same thing. We have a giant example of that in our sky: the Sun. The Sun is powered by the conversion of mass to energy.
Something from Nothing
Could dark energy show a link between the physics of the very small and the physics of the large?
But energy is supposed to have a source — either matter or radiation. The notion here is that space, even when devoid of all matter and radiation, has a residual energy. That "energy of space, " when considered on a cosmic scale, leads to a force that increases the expansion of the universe.
Perhaps dark energy results from weird behavior on scales smaller than atoms. The physics of the very small, called quantum mechanics, allows energy and matter to appear out of nothingness, although only for the tiniest instant. The constant brief appearance and disappearance of matter could be giving energy to otherwise empty space.
It could be that dark energy creates a new, fundamental force in the universe, something that only starts to show an effect when the universe reaches a certain size. Scientific theories allow for the possibility of such forces. The force might even be temporary, causing the universe to accelerate for some billions of years before it weakens and essentially disappears.
Or perhaps the answer lies within another long-standing unsolved problem, how to reconcile the physics of the large with the physics of the very small. Einstein's theory of gravity, called general relativity, can explain everything from the movements of planets to the physics of black holes, but it simply doesn't seem to apply on the scale of the particles that make up atoms. To predict how particles will behave, we need the theory of quantum mechanics. Quantum mechanics explains the way particles function, but it simply doesn't apply on any scale larger than an atom. The elusive solution for combining the two theories might yield a natural explanation for dark energy.
Stranger and Stranger
Most of the universe seems to consist of nothing we can see. Dark energy and dark matter, detectable only because of their effect on the visible matter around them, make up most of the universe.
We do know this: Since space is everywhere, this dark energy force is everywhere, and its effects increase as space expands. In contrast, gravity's force is stronger when things are close together and weaker when they are far apart. Because gravity is weakening with the expansion of space, dark energy now makes up over 2/3 of all the energy in the universe.
It sounds rather strange that we have no firm idea about what makes up 74% of the universe. It's as though we had explored all the land on the planet Earth and never in all our travels encountered an ocean. But now that we've caught sight of the waves, we want to know what this huge, strange, powerful entity really is.