Cosmological constant problem

In cosmology, the cosmological constant problem or vacuum catastrophe is the substantial disagreement between the observed values of vacuum energy density (the small value of the cosmological constant) and the much larger theoretical value of zero-point energy suggested by quantum field theory. Depending on the cutoff of Planck energy and other factors, the quantum vacuum energy contribution to the effective cosmological constant is calculated to be between 50 and as many as 122 orders of magnitude greater than has actually been observed, a state of affairs described by physicists as "the largest discrepancy between theory and experiment in all of science" and "probably the worst theoretical prediction in the history of physics".

Source: Wikipedia — Cosmological constant problem (CC BY-SA 4.0)