After Uranus was discovered, astronomers saw deviations in its orbit that could be caused by the gravity of another large planet. Using those deviations as a guide, Neptune was discovered soon after.

There were still some deviations in the orbits of both Uranus and Neptune, however, so there was a search for a 9th planet.

This search found Pluto, which is too small to have much of an effect on the orbits of Uranus and Neptune. There have been several surveys of the sky for additional large outer planets in both visible and infrared wavelengths, and the searches turned up nothing.

Because of this, the general conclusion is that there is not another large planet in the outer solar system.