Our universe expands, and this expansion is accelerating. Current consensus is to attribute this acceleration to a mysterious form of energy: dark energy. This dark energy density is very tiny and therefore only notable at cosmic length scales. When expressed in natural units, the cosmic dark energy density has a value of 10-123. This tiny value presents a big mystery. Straightforward estimates for the dark energy density based on quantum field theoretical considerations result in values (again in natural units) close to unity.