Their existence is canon. Their presence in the Hydral system is debateable. It's like how the AI War ended. We know that it did end. But how we don't know, and whether the humans won it at all... (well okay they did probably, if only so we can go Through a Mirror, Darkly at some later Arcen game point).
Maybe a future TLF expansion will add a small Obscura incursion event that can serve to explain why they don't show up in the base game. Until then, it's a mystery~
Chris if you see this, would it be untoward of me to Mantis suggest calling them a "particulate based" life form rather than particle based?