They were capturing extra-dimensional beings and using their corpses to power their starship so they could get home faster.

Just say “Body Thetans,” it’s okay to admit that some Starfleeters are secretly Scientologists.

Got a question for ye. What weighs more: an isogram of dilithium, or an isogram of mycelium? Take your time.

So it sounds like Voyager is set in the Warhammer 40k universe then, since the Warp in that lore is a roiling storm of madness and joy of all the thrashing souls of everything sentient that ever lived (and sometimes not sentient, such as literal concepts of thought like hatred, and even forgotten gods of long dead ancient religions) since the beginning of time.