Short answers, pulled from the story.
Albert Einstein introduced the cosmological constant labeled with the Greek letter Lambda (Λ) into his field equations of general relativity. He intended for this term to balance gravity with repulsive forces and prevent the universe from collapsing under its own weight.
Measurements of Type Ia supernovae provided the first direct evidence for dark energy in 1998. Two teams published observations claiming the universe's expansion is accelerating, leading to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess receiving the 2011 Nobel Prize in Physics for this discovery.
The High-Z Supernova Search Team released data showing distant supernovae were fainter than expected because they serve as standard candles with constant luminosity. Comparing distance to redshift reveals how fast the supernova recedes and indicates that the universe expanded more in its latter half than in its former half.
Scientists propose several explanations including a cosmological constant representing constant energy density filling space homogeneously and Quintessence models suggesting a dynamical field varying in space and time. Other theories include Phantom dark energy where energy density increases with time and Modified gravity theories attempting to eliminate the need for dark energy entirely.
If acceleration continues indefinitely, galaxies outside the Local Group will recede faster than light and cross a cosmological event horizon measuring about 16 billion light years from Earth. Most galaxies will disappear while Planet Earth and the Milky Way remain undisturbed until the scenario leads to heat death for the Local Group.