Final answer:
The disk structure of the Milky Way is primarily the result of collisions with other galaxies over time, specifically the accretion of a satellite galaxy. This process stirred up the orbits of stars and gas clouds in the thin disk, causing them to form the thick disk.
Step-by-step explanation:
The disk structure of the Milky Way is primarily the result of collisions with other galaxies over time. Calculations indicate that the thick disk of the Galaxy may be a product of one or more such collisions. The accretion of a satellite galaxy would stir up the orbits of stars and gas clouds originally in the thin disk and cause them to move higher above and below the mid-plane of the Galaxy, creating the thick disk.