  The cosmological constant was proposed 100 years ago in order to make the
model of static Universe, imagined then by most scientists, possible. Today it
is the main candidate for the physical essence causing the observed accelerated
expansion of our Universe. But, as well as a hundred years ago, its nature is
unknown. This paper is devoted to the story of invention of $\Lambda$ by Albert
Einstein in 1917, rejection of it by him in 1931 and returning of it into the
science by other scientists during the century.
