Idea that some time ago (13. 7 ga ago), everything is compressed into small parts. Then, the universe began to expand rapidly. As it expanded, there were lumps (some places were more dense than others) Higher density places had higher gravity, which attracted more particles, and became even denser. These lumps became all the different galaxies in the universe, composed of billions of stars. Explosion from an infinitesimally small point, producing irregular distribution of hydrogen, helium and other light elements. A nebula forms from hydrogen helium and heavier elements left from the big bang, and previous star cycles. The nebula condenses into a swirling disc, with a central ball surrounded by rings. Ball at the center grows dense and hot enough for fusion, becoming the sun. In the rings, dust particles collide and stick together, forming planetesimals (tiny rocky bodies) Planetesimals grown into the proto-earth, interior heats up (materials collider, and decay of radioactive material) and becomes soft.