Just read that there are perturbations of stuff in the outer solar system that suggest there are Earth sized planets far out from the sun, in the dark.

When the same sort of effects are seen at the galactic scale, cosmologists use Dark Matter to explain where the unknown force is coming from.

So why when its small scale (solar system) do they use Newtonian physics and on the galactic scale use hypothetical Dark Matter ? Where is the cut off ?

Also, could Dark Matter be just dark matter, stuff that does not emit radiation and sits in the dark ? More and more dark stuff seems to be discovered every year, its odd that this option is not followed up, whereas Dark Matter seems to get all the attention even though there is no experimental evidence for it.