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.
Not a stupid question.
Beyond the too far to reflect light aspect, the evidence we do have is gravitational effects to other bodies we can see and track. So there is evidence that something is out there. It’s just a matter of what and exactly where.