Short answers, pulled from the story.
Einstein added the cosmological constant in 1917 to prevent his general relativity equations from predicting a collapsing universe. The scientific consensus at the time assumed the universe was static, and without the constant, gravity would cause any non-expanding universe to contract. He later described adding it as his "biggest blunder" after Edwin Hubble confirmed the universe was expanding.
Observations of type Ia supernovae in 1998, by teams led by Saul Perlmutter, Brian Schmidt, and Adam Riess, showed that the universe's expansion is accelerating rather than slowing down. This required a positive cosmological constant to explain, reversing decades of scientific consensus that the constant was zero. Perlmutter, Schmidt, and Riess received the Nobel Prize in Physics in 2011 for this discovery.
The cosmological constant problem is the enormous discrepancy between the vacuum energy predicted by quantum field theory and the value actually measured through cosmological observations. The theoretical prediction exceeds the observed value by approximately 120 orders of magnitude, a gap physicists have called "the worst theoretical prediction in the history of physics." No known theory satisfactorily explains why the observed value is so small.
The Planck Collaboration published results in 2018 estimating that the dark energy density parameter is 0.714, meaning roughly 68 percent of the universe's total mass-energy density is attributable to dark energy. This fraction is not constant over cosmic time; as the universe expands, dark energy grows with the volume while the amount of matter remains fixed.
In 1987, Steven Weinberg predicted using the anthropic principle that the cosmological constant would be less than a hundred times the then-accepted value. He refined this in 1992 to between five and ten times the matter density of the universe. In 1995, Alexander Vilenkin further refined the prediction to about ten times the matter density, approximately three times the value later measured.
The cosmological constant is a central component of the Lambda-CDM model, which is the current standard model of cosmology. It serves as the simplest explanation for dark energy, the force driving the accelerating expansion of the universe. Studies assuming the cosmological principle attribute around 68 percent of the universe's mass-energy density to the dark energy described by the cosmological constant.