What is Dark Energy?

With all the talk lately about Dark Energy, I wanted to fully understand what it was. Going to Wikipedia, it states: In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. [1] Assuming the existence of dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for almost three-quarters of the total mass-energy of the universe. Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously,[2] and scalar fields such as quintessence or moduli, dynamic fields whose energy density can vary in time and space. In fact contributions from scalar fields which are constant in space are usually also included in the cosmological constant. The cosmological constant is thought to arise from the vacuum energy. Scalar fields which do change in space are hard to distinguish from a cosmological constant, because the change may be extremely slow. High-precision measurements of the expansion of the universe are required to understand how the speed of the expansion changes over time. The rate of expansion is parameterized by the cosmological equation of state. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today. Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model" of cosmology because of its precise agreement with observations. As we hear more, I will try to update my postings.