Why was the Milky Way’s spiral structure difficult to detect from Earth?

The Milky Way’s spiral structure was difficult to detect from Earth primarily due to the solar system's interior location within the galaxy and the presence of obscuring interstellar dust, which makes the structure challenging to observe using optical methods.1

Because this dust scatters and absorbs starlight, astronomers were unable to confirm the galaxy's spiral nature until 1953, when reliable distances to stellar associations were first obtained. Today, this structure is more easily discerned through radio mapping of neutral hydrogen or molecular clouds, as these emissions can be detected through the dust that blocks visible light.12