  The cosmological constant, also known as dark energy, was believed to be
caused by vacuum fluctuations, but naive calculations give results in stark
disagreement with fact. In the Casimir effect, vacuum fluctuations cause forces
in dielectric media, which is very well described by Lifshitz theory. Recently,
using the analogy between geometries and media, a cosmological constant of the
correct order of magnitude was calculated with Lifshitz theory [U. Leonhardt,
Ann. Phys. (New York) 411, 167973 (2019)]. This paper discusses the empirical
evidence and the ideas behind the Lifshitz theory of the cosmological constant
without requiring prior knowledge of cosmology and quantum field theory.
