I've heard this referred to as a "dark sector", or "hidden sector":

https://en.wikipedia.org/wiki/Hidden_sector

The scary thought is that, were it true that there is voluminous richly-interacting dark matter, we would be the actual dark sector.

"Imagine a universe, like ours, overlapping ours, except where some of its matter spontaneously rips itself apart, and other matter can be mashed together if you squeeze it hard enough." "That's nightmare fuel! Fortunately it's probably impossible, so far as we can tell."

I can imagine other scary thoughts, though. Ever read any Warhammer lore?