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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 Ways corona , where much of the galaxys mass must be located, is a tiny fraction of the coronas mass. Thus, most of the Milky Ways 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 Ways 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. At present, therefore, the best explanationthough still quite tentativefor the darkness of the corona is that the corona is composed mainly of old, burned-out stars. 17. The passage as a whole is primarily concerned with analyzing a current debate criticizing a well-established theory showing how new facts support a previously dismissed hypothesis stating a conclusion and adducing evidence that may justify it contrasting two types of phenomena and showing how they are related 18. According to the passage, a bright part of a galaxy typically includes | 
