The universe is not only expanding – it is accelerating outward, driven by what is commonly referred to as “dark energy.” The term is a poetic analogy to label for dark matter, the mysterious material that dominates the matter in the universe and that really is dark because it does not radiate light (it reveals itself via its gravitational influence on galaxies). Two explanations are commonly advanced to explain dark energy. The first, as Einstein once speculated, is that gravity itself causes objects to repel one another when they are far enough apart (he added this “cosmological constant” term to his equations). The second explanation hypothesizes (based on our current understanding of elementary particle physics) that the vacuum has properties that provide energy to the cosmos for expansion.
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