Massive, hidden planet in our solar system?

Is there a massive, hidden planet in our solar system, bigger even than the largest known planet Jupiter?  Or perhaps a red dwarf star that is a sister to our Sun yet which until now has been undetected?

Far far beyond the orbit of Neptune lie what are called trans-neptunian objects.  Way out there is a region called the Oort Cloud where it is believed trillions of objects exist.  The only thing is, at such a far distance, the region of space is exceedingly vast, so even though there are so many objects, they are still extremely far apart.