When Albert Einstein, Willem de Sitter, Alexander Friedmann, Georges-Henri Lemaître, and others were working on the nature of the universe in the early twentieth century, Einstein introduced a mathematical term into his equations to keep a balance between cosmic expansion and gravitational attraction. This term became known as the “cosmological constant,” and seemed to represent an unseen energy that emanated from space itself.

After Edwin Hubble and other astronomers showed that the universe was indeed expanding, the cosmological constant no longer appeared to be necessary, and so it was not seriously considered again for decades. Then, starting in the 1990s, a series of discoveries suggested that the “dark energy” represented by the cosmological constant does indeed exist. Current measurements indicate that the density of this dark energy throughout the universe is much greater than the density of matter—both luminous matter and dark matter combined.

Though astronomers have measured the presence of this dark energy, we still have no idea what causes this energy, nor do we have a clue what this energy is made of. The quest to understand the cosmological constant in general, and dark energy in particular, addresses one of the great unsolved mysteries in astronomy today.