Final answer:
The difficulty in mapping the Milky Way in the visible spectrum lies in the dust within the galactic plane that obscures the light from distant stars, similar to smog on Earth. Astronomers use other forms of light, like radio waves, to map the galaxy's structure despite the dust.
Step-by-step explanation:
It is challenging to map the Milky Way in the visible spectrum because close to the galactic plane the light from distant stars is blocked by dust. This dust, accumulating over thousands of light-years, creates a veiling effect much like smog does on Earth, obscuring the stars that are further away. While comets and asteroids within the solar system can obstruct our view of individual stars, they are not the main reason distant stars are hidden in the galactic map. Additionally, it is not the expansion of the universe that hides distant stars within our galaxy, since this effect is more pertinent to objects far beyond the Milky Way's boundaries.
Astronomers have found ways to circumvent the limitations posed by interstellar dust by using other forms of light that can penetrate the dust layers, such as the 21-cm radio waves emitted by cool hydrogen. Such innovations have enabled the creation of a more accurate map of the Milky Way, despite the challenges presented by being located within its bounds.