genocide on a galactic scale. send an information virus that makes every alien species peaceful so they won't go hunting for you, if they can't harm another living thing the species will effectively die off (or reduce to a tiny non threatening non thriving species) within 200 years as the show has demonstrated.
it could be on purpose, it could be a weird mutation accident, it could be a peaceful intention that was warped by a third party, it could be created by an artificial intelligence to wipe out organic life, a million other options. The core idea is that since space is huge and FTL doesn't exist, there is no point in "going" anywhere (1000+ years to closest star, time dilation issues) but to protect your own empire you make sure no one else exists to come knocking on your door.
I have a feeling Vince Gilligan didn't really think it out all that far, its just an interesting idea and I wouldn't be super shocked if the show never really deals with it. I'm sure from a story telling perspective Its more interesting to keep it mysterious and never find out who the aliens were or why, and just show how humans deal with it. I don't expect this show to turn into 3 Body Problem, you can watch that show/books if you like the dark forest idea.