GRE短阅读一道答案求分析~

GRE短阅读一道答案求分析~
文章：It is now established that the Milky Way is far more extended and of much greater mass than was hitherto thought.However,all that is visible of the constituents of the Milky Way’s corona (outer edge),where much of the galaxy’s mass must be located,is a tiny fraction of the corona’s mass.Thus,most of the Milky Way’s outlying matter must be dark.
　　Why?Three facts are salient.First,dwarf galaxies and globular clusters,into which most of the stars of the Milky Way’s corona are probably bound,consist mainly of old stars.Second,old stars are not highly luminous.Third,no one has detected in the corona the clouds of gaseous matter such as hydrogen and carbon monoxide that are characteristic of the bright parts of a galaxy.
According to the passage,a bright part of a galaxy typically includes
A.dwarf galaxies and cluster of stars
B.a balaced mixture of old and new stars
C.a large portion of the galaxy's mass
D.part of the corona of the galaxy
E.gases such as hydrogen and carbon monoxide
答案选的是E.我选了C.我的依据是all that is visible ...看的见的部分应该就是明亮的部分咯,where much of the galaxy's mass must be located,说明bright part是galaxy's mass咯?
我想知道答案出处no one has detected in the corona the clouds of gaseous matter such as hydrogen and carbon monoxide that are characteristic of the bright parts of a galaxy.的意思.我个人觉得认为这句话表达的是hydrogen and carbon monoxide不具有星系中明亮部分物质的特征.也就是说这两种气体不是明亮的部分咯?那么答案为什么还选E呢?