Specifically, what were the observations that led us to discover what kind of galaxy the Milky Way is? What kind of galaxy is the Milky Way? Is it a typical one?
2. Corrected_text: What was the first indication based on observations of distant stars and gas in our galaxy that much of the matter of the Milky Way was invisible; that is, it has mostly stuff that is not luminous and thus became called dark matter?
3. Title_with_topic:
1. Understanding the Milky Way: Observations and Classification
2. Dark Matter: The Invisible Component of the Milky Way