The phrase "dark energy" sounds mysterious to me, sort of "ghostly". However, as explained by Wikipedia, dark energy is believed to be the energy that causes the acceleration of the expansion of the universe
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory 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 73% 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, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant is physically equivalent to vacuum energy. Scalar fields which do change in space can be difficult 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 expansion rate changes over time. In general relativity, the evolution of the expansion rate is parameterized by the cosmological equation of state (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space). Measuring the equation of state for 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. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.
A 2011 survey of more than 200,000 galaxies appears to confirm the existence of dark energy, although the exact physics behind it remains unknown.
Here are additional science reports about dark energy.
Researchers have identified a burst of high-energy radiation known as 'dark lightning" immediately preceding a flash of ordinary lightning. The new finding provides observational evidence that the two phenomena are connected, although the exact nature of the relationship between ordinary bright lightning and the dark variety is still unclear, the scientists said.
To explain the acceleration of the expansion of the universe, astrophysicists have invoked dark energy -- a hypothetical form of energy that permeates all of space. A popular theory of dark energy, however, does not fit new results on the value of the proton mass divided by the electron mass in the early universe.
"But with our new work we're more confident than ever that this exotic component of the Universe is real -- even if we still have no idea what it consists of."
These stellar explosions helped astronomers conclude more than a decade ago that dark energy is accelerating the expansion of the universe, and this week earned the discoverers -- including UC Berkeley physicist Saul Perlmutter -- the 2011 Nobel Prize in Physics. But what caused them was a mystery. Many astronomers thought white dwarf stars were pulling matter from their normal stellar companions and growing so fat they exploded.
The most widely accepted property of dark energy is that it leads to a pervasive force acting everywhere and at all times in the universe. This force could be the manifestation of Einstein's cosmological constant, which effectively assigns energy to empty space, even when it is free of matter and radiation. Einstein introduced the cosmological constant into his theory of general relativity to accommodate a stationary universe, the dominant idea of his day. He later considered it to be his greatest blunder after the discovery of an expanding universe.