Although Most Astronomers Assume Dark Matter Really Exists, There Is

Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it?

A) There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.
B) The so-called dark matter is really just ordinary stars that are enshrouded in clouds of dust.
C) We could just be having a hard time understanding the observations because they involve very distant galaxies.
D) There could be something wrong with our understanding of how atoms produce light.