The cosmological constant was a physical concept proposed by Albert Einstein which was needed to understand the expansion or contraction of the universe.
In 2367, a Cytherian-enhanced Reginald Barclay discussed the cosmological constant with a holographic Albert Einstein. Barclay wondered how Einstein included quantum principle into the Theory of General Relativity without adjusting the constant much more, which according to Einstein would lead to twenty-six dimensions instead of ten. (TNG: "The Nth Degree")
According to Jack in 2375, a massive spherical grid of subspace field generators could be used to manipulate subspace to reduce the mass of the universe and therefore change the cosmological constant. This would avert the collapse of the universe. (DS9: "Chrysalis")