From the introduction:
Greetings, Jumper. In the year 2112, the Forerunner exploration craft came in contact with a mysterious alien life form hidden in another dimension. In 2120, Forerunner returned with a small high-energy living organism. Both creatures were named Bydo. Several technological developments followed. In 2134, contact with a laboratory situated near Jupiter involved in the development of Bydo-related weaponry was lost. 3 million meters of space appeared to be missing, and a larger Bydo fragment was retrieved. In 2163, the Bydo made their move. War began and the outer rims of the solar systems came under constant attack. An R-9A Arrowhead squadron was deployed in Bydo Mission I to defend humanity soon after. While this information would only be discovered a fair bit after your stay here is over, I suppose it wouldn’t hurt to explain the origins of the Bydo.