With the current news about the possibility of a planet in the Oort cloud, I was thinking, why is the matter in the Oort cloud distributed so? It makes sense that inside the planet region there is not a huge concentration of comets and meteors, but why is the matter inside the cloud itself not agglomerating? It seems like there is enough matter to form not one or two, but many more planets.
Simple answer: the material is just too spread out.
Forming a planetary-size object takes LOTS of collisions to build up enough matter to start gravitationally attracting the surrounding stuff. The Oort cloud is very far from the sun (starting at ~1000 AU, compared to a maximum of ~50 AU for Pluto), meaning that the icy fragments are moving very slow in their orbits. Furthermore, the material is spread out over a huge volume, especially since the Oort cloud is a spherical shell (the matter is spread in 3 dimensions), as opposed to the 2-d disk that makes up the vast majority of matter in the Solar system (Sun, Planets, Asteroid & Kuiper Belts). The timescale for these objects to accrete to any graviationally significant size is far beyond the lifetime of the Solar system (or even the universe).