Short answers, pulled from the story.
Dark energy is a proposed form of energy that drives the accelerating expansion of the universe. It makes up 68% of the total mass-energy of the observable universe, with dark matter contributing 27% and ordinary matter only 5%.
Dark energy's existence was established through supernova observations published in 1998 by the High-Z Supernova Search Team and in 1999 by the Supernova Cosmology Project. Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess received the 2011 Nobel Prize in Physics for their leadership in the discovery. The term dark energy was coined by cosmologist Michael S. Turner in 1998.
Einstein added the cosmological constant to his equations of general relativity to produce a static universe, using it to balance gravity. He later called this his greatest blunder after Edwin Hubble's 1929 observations showed the universe is expanding. The cosmological constant is now the simplest candidate explanation for dark energy.
A cosmological constant represents a fixed energy density of empty space that remains uniform across time and space. Quintessence is a dynamical scalar field whose energy density can vary in both time and space, and it generally predicts a slightly slower acceleration of expansion than the cosmological constant.
In March 2025, the Dark Energy Spectroscopic Instrument collaboration announced evidence that dark energy is evolving rather than constant, with statistical significance ranging from 2.8 to 4.2 sigma. The results suggest the density of dark energy is slowly decreasing with time and may be about 10% lower now than it was 4.5 billion years ago.
The Big Rip is a scenario in which an ever-growing dark energy force tears apart galaxies, solar systems, and eventually individual atoms, ending the universe. It is predicted by phantom dark energy models, in which the energy density of dark energy increases with time rather than remaining constant.