Possible theories about the Flood.

What if the Flood are not parasitic aliens at all, but rather an otherworldly entity from another dimension? What if the Forerunners accidentally opened a portal to their dimension which resulted in making their way on several planets, the likes of which are unknown in the Halo universe that led to the creation of the rings?

Or what if the Flood are(or were)a by-product of evolution?


Feel free to speculate.