Dark energy
Quintessence
In cosmology, a hypothetical new 'element,' distinct from any normal matter (either baryonic or not) or radiation, intended to explain the observed ever accelerating expansion of the Universe. Quintessence can have several types and differs from the cosmological constant in that it can vary in space and time. In modern physics, the four known "elements" are the baryons (proton, neutron, etc.), the leptons (neutrinos, electrons, etc.), the photon, and the hypothetical non-baryonic matter, which is thought to be 80% of the total matter in the Universe. The quintessence field is a possibility which can be confirmed or disproved by measurements of the dark energy value at different redshifts. In some models, the quintessence is fine-tuned to explain both the cosmological constant problem and the inflation in the very early Universe.