The Dark Energy Mystery

In 1998, two teams of scientists measured the universe's rate of expansion, expecting to find the gravity of the Cosmic Web bringing the universe into a collapse. But instead, they found that the universe's expansion appears to be accelerating- having expanded more in the latter half of its life.
Both teams agreed upon the existence of a new type of energy field, known as Dark Energy, to drive this acceleration, and in doing so, they revived one of Einstein's long-rebuked ideas- the Cosmological Constant.