There are odd things about the orbits of trans-Neptunian objects that suggest ( to some) that there might be an undiscovered super-Earth-sized planet a few hundred AU from the Sun..
We haven’t seen it, but then it would be very hard to see. The underlying reason is simple enough, but I have never seen anyone mention it: the signal from such objects drops as the fourth power of distance from the Sun. Not the second power, as is the case with luminous objects like stars, or faraway objects that are close to a star. We can image close-in planets of other stars that are light-years distant, but it’s very difficult to see a fair-sized planet a few hundred AU out.