First, let me explain that I claim no originality whatsoever in this setting. It is but a mere collage of different things that I like; you'll soon find plenty of references to Stargate, Galactica, Aliens, Riddick... I have not cared about changing the names of those "copied" things, although I will twist them every now and then, so they fit better together and contribute to a grim atmosphere.
Now let's get started with history:
At the beginning of the XXI century, a routinous archaeological excavation in Egypt discovered an ancient device of very advanced technology that was named "stargate". This stargate allowed for the teleportation from Earth to different planets containing working stargates. It was soon realized that most, if not all, inhabitable planets in the galaxy had these devices. At the same time, mankind learnt that Earth had belonged to the Gouald Kingdom, providing both hosts and slaves for the parasitic goualds.
These, infuriated by seeing Earth bound humans had become so advanced and that they were promoting rebellion amongst the goualds' human slaves, tried to stop their exploring the galaxy. Yet, despite their superiority, the guerilla-like nature of the conflict allowed the humans to resist against their new enemies, and even have some significant victories. All the while, alien technology falling in human hands allowed for a scientific boost. Moreover, humans also met the Asgards, another alien race that had historically opposed the goualds.
Finally, in the last years of XXI century, humans won due respect and were accepted in the galaxy-wide Pact of Major Races. Among other things, it meant that they were not the possesion of the Gouald Kingdom, and so they had to offer freedom to all their slaves, and search for a different species to parasite from then on. Yet half of the slaves chose to continue with their lives; and it was not such a bad idea, because all freed humans had to migrate to an already overpopulated Earth, whose population doubled within a decade.
The next century is commonly referred to as the "lost" one. Extreme starvation plagues very soon led to conflicts, struggles, and war. Even though mankind avoided falling in a World War Three, more than fifty different wars erupted (which had the dubious benefit of keeping population controlled). The little resources that were not spent on the military were destined to the space race, now more a space sprint searching for habitable planets.
Of course, being acknowledged as a Major Race didn't include the posession of any useful system. Mankind had to settle for seven systems that had planets that were inhabitable but could be, in theory, terraformed. This process was started as soon as possible, medium of XXII century. The living conditions of the pioneers were horrible, but actually not much worse than what Earth was like those days; and official propaganda worked hard to give a halo of heroism to the pioneers.
By the early XXIII century terraforming had been completed, and massive migrations began. Yet mankind had to suffer one more misfortune. Goualds were still hostile towards humans, and they appealed to an obscure passage in the Pact to claim the stargates of human planets as their own. After a long legal battle humans only kept the Earth stargate, losing the ones from the seven colonies. Starship technology allowed only for slower than light speeds, and that was clearly insufficient for the needs of mankind. And building stargates was still far from human science. An alternative teleportation method was created, one that every other race in the galaxy had banned millennia ago because of its risk: the creation of controlled mini black holes, with which the space continuum is pierced. Of course, those maneuvers can't be made anywhere near a celestial object, because there's always a minimun gravity leak. And every now and then, ships get lost in the void of "the other side" when the black hole generator malfunctions. And what's worse, some of them later return.
Finally established and with difficulties overcome, humans had truly become a spacefaring race. Scientific knowledge was free to develop again, and great attention was put in genetics. The old medical motto of "prevention better than curing" was put to an extreme with the genetic manipulation of human ADN to make our races stronger and far more resilient. This investigation was not a bed of roses, and many disastrous failures and much ethical and political debate took place before it was finally incorporated into everyday life. Also, colonies may had been terraformed, yet they all had specific particularities; in order to compensate them, more genetic engineer "tuned" the colonies' inhabitants to their habitats, around the turn of the XXIII century into the XXIV.
Comparatively, this last century has seemed the more placid one in a lot of time. Which doesn't mean nothing has happened.
Externally, the race of the Ancients, founders of the Pact of Major Races and developers of most of the technology spread throughout the galaxy, have been declared officially extinct, after millennia since their sudden and misteryous disappearance. Sliggs, a proud and militaristic race of humanoid reptilians, whose empire is bigger than any other one, are entangled in an internal war against their own cybernetic creations; and it is believed they are losing it.
Meanwhile, internally, human tendency towards independentism is causing tension in the political relations of Earth and Colonies. The fact that the latest are significantly close to each other, whereas the former stands very far apart is joined by the totalitarian ways of directing politics from the capital. In the other hand, this XXIV century has seen a rise in economical corporation power unmatched since the fierce neo-liberal years of the XX century. Enterprises buying political influence has only lead to more destabilization and corruption spreading like a wild fire, now that the XXV is already here.