Observations of distant galaxies show that they are moving away from each other, including from our Galaxy - the observable Universe is expanding. In the distant past, there were no separate galaxies, separate celestial bodies, all the matter of the Universe was very dense and hot. The expansion of the universe is a consequence of a giant explosion that occurred about fifteen billion years ago. This explosion was very unusual and due to quantum effects in the superdense matter of the Universe. The book tells how modern science solves this greatest mystery of nature.