However, there's no way that would be enough to account for the majority of the missing mass. If it were baryonic matter, there'd be a lot more light extinction from all the clutter in between the stars.
That's not even true for planets ORBITING stars, why would you expect it to be true of large cold planets in interstellar space?
Excitation is detectable for some types of matter in a diffuse state -- atomic hydrogen, for example -- which can be detected as it receives and then sheds an electron/photon from nearby energy sources. Excitation in a planetary atmosphere is harder to detect since 1) the planet itself re-absorbs alot of that energy and 2) it is a relatively small amount of energy being emitted from a VERY small point in distant space, below the resolution of most telescopes.
Not neccesarily brown dwarfs, but jupiter or saturn-mass rogue planets wandering the galaxy is likely to be a fairly common situation
, and some of those may be double or tripple planet formations orbiting each other. This would, of course, imply that most of the time a collapsing cloud of gas and dust would not achieve anywhere near enough density or temperature to actually fuse elements; even below brown dwarfs, there would be whole solar systems that form around a couple of large gas giants with no star to speak of. It's even possible that these cases are the vast majority of planetary systems in the galaxy and that they remain undetected only because our equipment for measuring gravitational lensing isn't precise enough to detect them.