What was the central scientific conflict known as the Great Debate centered around in the early 1900s?

Answer

The nature and location of spiral nebulae

The Great Debate focused on whether spiral nebulae were small gas clouds within the Milky Way or entirely separate 'island universes' located far outside our galaxy.

What was the central scientific conflict known as the Great Debate centered around in the early 1900s?
discoverygalaxyAstronomerMilky Way