A force proposed to account for the apparent acceleration in the expansion of the Universe, as deduced from observations of supernovae in distant galaxies. One proposed explanation for the acceleration of the expansion is a cosmological constant, but if such a constant exists its physical meaning is completely unknown; other possible explanations include a modification to our theories of gravity. Astronomers have generally preferred to explain the acceleration as a consequence of the fields which fill space, such as the electromagnetic field. According to quantum mechanics, every field should have a minimum energy called the vacuum energy, and this vacuum energy could account for the observed acceleration. The problem with this explanation is that the predicted effect of the vacuum energy on the expansion of the Universe is roughly 10¹²⁰ times greater than the acceleration actually measured. An alternative explanation for dark energy is a different type of field known as quintessence. The cause of the cosmological acceleration remains an unsolved question. See also Cosmology.