If I remember right, it was alluded to in 2 in the Quarian quests in that they found unusual amounts of dark matter in a star.

The premise was going to be the mass effect technology was CAUSING dark matter build up and that the Reapers had to purge advanced life everytime they mass developed the technology to stop all stars dying out and ending the galaxy.

I have this Pcgamer article from 2013 that goes into more detail, sounds a lot better than the ending we got.