All of space and its contents, the study of which is called cosmology. The Universe is thought to be around 13–14 billion years old, and is mostly empty space, dotted with galaxies for as far as telescopes can see. These galaxies are moving further apart as the universe expands. The standard theory of how the Universe came into being and evolved is the Big Bang theory of an expanding Universe originating in a single explosive event.

The most distant detected galaxies and quasars lie 10 billion light years or more from Earth. Apart from those galaxies within the Local Group, all the galaxies we see display redshifts in their spectra, indicating that they are moving away from us. The further we look into space, the greater are the observed redshifts, which implies that the more distant galaxies are receding at ever greater speeds.

This observation led to the theory of an expanding Universe, first proposed in 1929 by US astronomer Edwin Hubble, and to Hubble's law, which states that the speed with which one galaxy moves away from another is proportional to its distance from it. Current data suggest that the galaxies are moving apart at a rate of 50–100 kps for every million parsecs of distance (one parsec equals 3 × 1 013 km).

https://www.windows2universe.org Dramatic site containing lots of information about the Universe. It contains sections on the Earth and the Solar System, plus myths about the Universe.