Visual Timeline of the Universe

The young universe did not have a perfectly even distribution of energy and particles. These irregularities allowed forces to start to collect and concentrate matter. Accumulations started to develop ever more complicated structures. Concentrations of matter formed into clouds, then condensed into stars and the collections of stars we call galaxies. The way in which galaxies spin indicates that their visible portions of stars and diffuse gas and dust clouds known as nebulae constitute only one tenth of the total mass. The so-called "missing mass" could hold the key to the ultimate fate of the universe -- that is whether it expands forever or is pulled back together by the combined gravitational attraction of all of its mass.

Hubble Deep Field From the standpoint of the development of life, what matters is that each galaxy is a stellar factory, producing stars out of giant gas clouds; each star is a chemical factory, transmuting simple elements into heavier, more complex ones; and life is a collection of some of these complex molecules. Visible matter comes in a wonderful variety of galaxy forms, characterized by their distributions of stars and glowing or dark nebulae.