The book tells how for several centuries scientists have tried to figure out why it is dark at night. It turns out that this question is connected with the most general structure of our Universe - with whether it is finite in time and space or infinite, whether it is actually expanding and what it consists of. The book discusses in detail the basic observational facts underlying modern cosmology and the history of their discovery. For anyone interested in astronomy and cosmology - from high school students to specialists in other fields of science.