At one point there was a theory that there was another Earth, in the same orbit as ours, but diametrically opposite so that it was always hidden by the Sun.
There may actually be one. I'm not aware of anyone who has wasted expensive spacecraft time, energy and bandwidth to point an instrument in that direction to look.
If there was anything of any size there, we would be able to measure its effect on the rest of the planetary orbits.
Also, it would not be in a stable orbit, and would eventually either get ejected from the solar system, move into a different, stable orbit, or intersect with us (as Theia presumably did).
There may actually be one. I'm not aware of anyone who has wasted expensive spacecraft time, energy and bandwidth to point an instrument in that direction to look.