How exactly did this happen? That answer, a bit longer.
About 1000 years in the future, mankind has expanded their reach by exploring and settling colonies on other planets in other solar systems within the Milky Way, yet have never come in contact with any other sentient or living extra-terrestrial life, and the idea of their existence has become somewhat of a joke among explorers. Space exploration was going far beyond many thought would be possible, however, this wasn't enough for a pair of scientists. Humanity has always dreamed to explore the far-reaches of space, and with their current technology they weren't able to cover the ridiculously vast distances required to even leave their own galaxy. A gate is designed, known as the Marco-Erickson gate (named after its two inventors) that is able to transmit matter (and data) nearly instantaneously to a far corner of the universe.
The device is a pair of "gates." The first gate is able to shoot off the second gate to a distant point in space; however, there is no real way to aim it, or, once it's there, of knowing where it is. The gate, however, is attracted to mass so the gate has a higher probability of landing near systems. Once the second gate is placed, the first gate is able to transmit matter directly to the second gate, and the second gate to the first gate. In other words, having two gates is like having a portal, having one gate is like having a massive slingshot that you can't aim.
A group of settlers, lead by Commander Brewer, passed through this gate and began to build a colony. There was no way of knowing really where it is as the star orientation was obviously completely different, but that wasn't really their top concern. They found a planet near the gate, a part of a larger solar system. They name the nearby planet, as well as the system as a whole, Terminus.
Though humanity had never before encountered one, there was an alien race living on Terminus that the humans never noticed before they fled: a struggling race known as the Caer (Sah-EER) who were working on recouping their strength after a great conflict. Being fresh off of war, and having an alien race settling on their planet, they became worried and fled, but not before discreetly destroying the gate that brought the human threat to their home. With the gate destroyed, the humans become forever stranded on Terminus with no hope of ever returning. As the Caer rebuild their numbers, they watch the humans cautiously; monitoring the human's communications and gathering all the intel they can.