The universe contains millions of galaxies which are composed of dust, gas and stars. The Big Bang theory suggests the universe began 14 billion years ago. Our galaxy, the Milky Way, is spiral-shaped and contains the Solar System including eight planets that orbit the Sun. The Earth revolves around the Sun while rotating on its axis, producing day/night and the seasons. The Moon also revolves around the Earth over 28 days, displaying different phases as it is illuminated by the Sun throughout its revolution.