  An introductory account is given of the understanding of the structure of the
universe. At present the most plausible theory of the origin of the universe is
that it formed from the explosion of an extremely hot and dense fireball
several billion years ago. During the first few seconds after the big bang, the
energy density was so great that only fundamental particles (leptons, quarks,
gauge bosons) existed. As the universe cooled and expanded after the big bang,
nuclei and atoms formed and condensed into galaxies and stars and systems of
them. The fundamental particles and a wide range of gravitational aggregates of
them constitute the small-scale and large-scale structure of the present
universe. The current knowledge of the elements of the structure of the
universe based on the standard big bang model and the standard model of
fundamental particles is considered.
