Evidence of Dark Energy Prior to its Discovery

This is a review and statistical analysis of the evidence supporting the existence of a cosmological constant in the early 1990s, before its discovery made with distant supernovae in 1998. The earlier evidence was derived from newly precise measurements of the Universe, including its mass density, the Hubble constant, the age of the oldest stars, the filamentary large-scale structure, and the anisotropy of the cosmic microwave background. These measurements created tension for models assuming the cosmological constant was zero. This tension was alleviated by several insightful papers published before 1996, which proposed a cosmological constant that increased the expansion rate. Statistical analysis here shows that the probability of the cosmological constant being zero was demonstrably less than a few percent. Some models identified a best-fit value close to the modern estimate of Omega_Lambda ~ 0.7.