This hypothesized ninth planet (not you, sorry Pluto) might explain the unusual commonalities of extreme trans-Neptunian objects (ETNOs) 100s of AU from the sun. These ETNOs (such as dwarf planets and sednoids) have remarkably aligned orbits, suggest the existence of an undiscovered celestial body, dubbed Planet Nine, influencing them gravitationally.
The problem is the potential planet is so far that it would barely reflect any light. We have inferred from computer models that for known planet and object orbits in our neck of the woods, there needs to be a planet there to create those orbits.
We just can’t see it because it’s so far out there, it’s really really dark, and it doesn’t pass in front of a star to see the dimming effect that would let us know it’s there. If we knew where to exactly point telescopes, it’s possible with enough study we could say it’s really there.
That makes sense. Thanks for the answer!