SUMMARIZE 
1. Exploration and Colonization of the Solar System: Initial voyages to Luna and the planets of the solar system. Stories of the first efforts to set up terrestrial bases on the planets. Stories of the first colonies on such worlds, their problems internal and external, their conflicts with the parent world (maybe even a war of independence), interplanetary commerce, spaceship trade lanes, space pirates, asteroid mining, the weird wonders of the Outer Planets.
2. First Wave Interstellar Exploration and Colonization: First interstellar flights. Starships that must travel centuries and contain generations descended from the original crews. Other planets of other stars. Contact with Terra is difficult at best. Lost colonies are typically founded during this era. Ben Bova calls this the "Marco Polo" stage of interstellar contact: adventure, strange tales, and artifacts. But no lasting political relations (for better or worse) with the neighbors.
3. Total or Limited Nuclear War on Terra: Forrest J. Ackerman calls it "atomigeddon". Widespread nuclear death on Terra. Fall of civilization. Mutants. Political map is wiped clean, most or all modern day nations are gone. Sometimes remote and marginal cultures will expand and take over the world, because they happen to be located far away from where the nukes fell. Eventual recovery.
4. Meeting With Aliens: First Contact, followed by second, third, fourth, and nth contact. Intelligences on extra-solar planets and our problems with them or against them. What happens depends upon whether the aliens technology level is lower, the same, or greater than humanity. And whether the aliens are friendly or hostile. Things can range from alien invasions to humans playing star-god with primitive aliens. There might be a Prime Directive imposed on human explorers.
5. Second Wave Interstellar Exploration and Colonization: See 2
6. Colonization of the Galaxy: Human colonies on other solar systems. Contact with Mother Terra, independence or dependence. Commerce: exploitation or otherwise. Go to The Tough Guide to the Known Galaxy and read the entry "COLONIZATION".
7. The Cycle of Empires
- 7a. Rise of the Galactic Empire: The rise of contact and commerce between many human-colonized worlds or many worlds of alien intelligences that have come to trust and do business with one another. For whatever reason the indepenent human and/or alien worlds unite. This can be for common defense, cultural reasons, economic reasons, or by conquest. The problem of mutual relations and the solution, usually in the form of treaties or defensive alliances. Implacable aliens in the cosmos who must be fought. The need for defense. The rise of industrial or financial or political powers, the eventual triumph of one and the establishment of a federation, a union, an alliance, or an autocratic empire of worlds, dominated usually from Old Terra.
- 7b. Galactic Empire at its Height: 
- 7c. Commerce between worlds an established fact, and adventures while dealing with worlds in and out of the Empire. The Pax Galactica reigns — a long period of peace and prosperity (at least on the surface). Technology is highly advanced. Civilization at it apex. During the Dark Ages, people will look back to this time as the Golden Age. The farthest planets, those of the Galactic Rim, considered as mavericks. The problem of aliens again outside the Empire, and outside our own galaxy. Politics within the government setup, intrigues, and dynasties, robotic mentalities versus human mentalities. Terraforming worlds for colonization. The exploration of the rest of the galaxy by official exploration ships (from the Survey Service), or adventurers, or commercial pioneers. Authors tend to avoid writing stories set in this period because it is very boring. This period varies depending upon the iteration, whether this is the First, Second, or latter Galactic Empire. The first is the most optimistic period. The Second Empire is generally wiser and more benevolent, but is also aware that empires can fall. In the Golden Age, the Second Empire was often also the Final Empire. Third and later empires are essentially the same setting as the Second Empire, but the higher number serves to imply an old galaxy, not locked in stasis.
- 7d. Galactic Empire Declines and Falls: Empire begins to decay. Intrigue and palace revolt. Breakaway planets. The alliance of worlds strained beyond its limits by rebellion, alien wars, decadence, corruption, scientific inability to keep up with internal or external problems. The rise of restless subject worlds. Outer provinces begin to revolt. Rim barbarians begin to invade. Decline, then loss of contact with farthest worlds, crumbling of commerce, failure of space lanes, distrust, finally worlds withdrawing into themselves as the empire/alliance/federation/union becomes an empty shell or is destroyed at its heart. Since Isaac Asimov showed the way, this period will resemble Edward Gibbon's Decline and Fall of the Roman Empire. Go to The Tough Guide to the Known Galaxy and read the entry "FALL OF EMPIRE".
- 7e. The Interregnum or Dark Ages: "The Long Night." Worlds reverting to pre spaceflight conditions, savagery, barbarism, primitive forms of life, superstition. Worlds taking to barbarian raids on defenseless isolated planets, hastening the downfall of knowledge. Interstellar trade and communication fails. Knowledge and technology is lost. Fragments of spaceflight, fragments of empire, some starships, some efforts to revive. Rise of petty wars and kingdoms. Thousands of years of loss of contact. Humanity in this period becomes indigenous to most of the habitable planets of the galaxy, forgetting origins. Evolutionary changes may take place. Alteration of form to fit differing world conditions — giant men, tiny men, water-dwelling men, flying men, mutations. Rairly this can end with the extinction of humanity. Go to The Tough Guide to the Known Galaxy and read the entry "INTERREGNUM".
- 7f. Renaissance: Rebirth of civilization. Interstellar trade and communications resume, and the seeds of a new Empire are planted. This step might be an overwhelming problem, because resource-wise you've got just one shot.
- 7g. Rise of a Permanent Galactic Civilization: Restoration of commerce between worlds. The reexploration of lost and uncontacted worlds and the bringing them back to high-technology, democratic levels. The efforts to establish trade between human worlds that no longer seem kin. Beating down new efforts to form empires, efforts which sometimes succeed and revert to approximations of the previous period, with similar results. Eventual rise of galactic harmony among intelligences. The exploration of other galaxies and of the entire universe.
- 7h. Delphic Age: Everybody wears togas. Galactic harmony and an undreamed of high level of knowledge leads to experiments in creation, to harmony between galactic clusters, and possible exploration of the other dimensions of existence. The effort to match Creation and to solve the last secrets of the universe. Sometimes seeking out and confronting the Creative Force or First Cause itself, sometimes merging with it. The end of the universe, the end of time, the beginning of a new universe or a new space-time continuum. Humanity ascends to a higher plane of existence or mysteriously vanishes/goes extinct.