We've all heard the on-again, off-again theories of another massive object in the vast reaches of the outer solar system. Some potential evidence to support the possibility are its affects on smaller dwarf planet bodies, with their distorted orbits.
I think most propose it to be a very large gas giant, if anything. I have also heard people propose a red dwarf star, though we would have surely seen this with our current telescopes.
What I'd like to know is, if a brown dwarf (something more massive and hotter than most gas giants) were the culprit, could it elude detection by our current means of observing the universe? They are hot objects, many of which produce some amount of light.