It got leaked at one point. If I remember correctly, the use of biotics and eezo was supposed to be destructive to space/stars/reality in general and continued use would eventually lead to the end of the galaxy and all life whether space-faring or not. The reapers were created to end advanced life, with its dependence on mass effect technology, before the effects were catastrophic in each cycle. The sun with irregular behaviour that Tali was investigating in ME2 was supposed to be showing early symptoms of its eventual destruction from these effects.