Abstract: In the general theory of relativity the space can not only be curved but it can also expand. This make universe expansion easily accessible also to wider audiences. One should just keep in mind that the special theory of relativity and conservation of energy have a limited validity. From the quantitative point of view, however, the observed expansion speed represents a cosmological mystery and is a subject of intense investigation in astronomy and physics. Special attention is paid in modern cosmology to the "dark energy" concept which might accelerate the universe expansion and explain thus many observations.

The lecture is of a popular character, suitable even for first-year students.