Rodents Among the Stars
Physics and Such
“Space” is not empty, but is rather filled with a substance called Aeroplasm, in the spirit of the historical scientific concept of “Aether”
Aeroplasm is indescribably thin, cold, and unbreathable to all except the birds, but not otherwise harmful. As such, most races only require a breathing mask (and possibly warm clothing) when exposed in the Aeroplasm.
The basic size of the universe is much smaller, with the default size of the Universe being
3 Size Ranks smaller than usual. As such, creatures of Size Rank -3 (Rats, for instance) are considered mechanically to be Size Rank 0.