Does anyone know if there's an upper limit on the size of a spiral? It would have to be limited by the size of it's dark matter halo, correct? Had a brief stint in cosmology in grad school, still find this stuff cool.

Faraday does not remember Desmond because he performed time travel experiments on himself and this messed up his memory. It's documented on his notebook: at the end of one episode he reads from it "...if something happens, Desmond Hume will be my constant."