Universe history. Artwork of the universe's history as a series of 8 circles. Time began about 15 billion years ago when a huge explosion (Big Bang) created all the universe's mass at a single point (top left). Circle 2 shows the hot & dense early universe. Circle 3 shows the formation of quark particles. Circle 4 shows quarks colliding to form protons and neutrons. Circle 5 shows the universe after 1 second, when electron & positron particles appeared. Circle 6 shows neutrons & protons combi- ning to form atomic nuclei. Circle 7 shows elect- rons combined with nuclei to form atoms (after 300,000 years). Circle 8 shows the universe today, where atoms have condensed into stars and planets.