Matter/antimatter vs "regular"/"dark" matter
One of the mysteries of physics is why matter dominates over antimatter. There are theories but we can't point to anything and say with absolute certainty we know that's the reason.
We believe dark matter dominates and what we consider regular matter is less abundant. Since dark matter and regular matter don't appear to interact (other than via gravity) there could be a whole other universe superimposed upon ours with its own stars, planets and life that we're currently oblivious to - and vice versa. At least until a certain level of technology is reached where it might become possible to peel back the veil.
Maybe aliens keep visiting us from the dark matter world via regular matter ships they've found out how to build and probe our asses because life forms made of our type of matter are such a rarity! Or maybe Nibbler isn't the only one with turds that can be used to fuel spaceships.