Quintessence

Quintessence or dark energy is a hypothetical form of energy postulated to exist in order to explain observations of an accelerating universe. This energy would act like a vacuum pressure, pushing things apart. Other attempts to explain these recent observations involve a non-zero cosmological constant, which has the same effect. Indeed, sometimes quintessence is said to result in a non-zero cosmological constant, and conversely a non-zero cosmological constant describes the effects of quintessence.

---

Among the classical elements, quintessence (meaning "fifth element", along with earth, air, fire, and water) was another term for aether.