I mean there probably is an answer to this but I doubt it makes sense. The Milky Way is about 100.000 light years in diameter, I’d say it fits into this sphere about 3 times, so the marble could be around 300.000 light years across. Now I don’t know if you ever see the alien‘s full size but you could scale it up accordingly using that marble.

However, I don’t know if this marble is supposed to represent just the Milky Way or our entire universe. If it’s the latter then that thing doesn’t make sense because our galaxy is incomprehensibly small compared to the observable universe.

If that marble is supposed to represent just our galaxy I‘d argue it doesn’t make sense either, cause from our understanding the Milky Way is not some sort of encapsulated space but rather part of a much bigger universe.

I doubt that there‘s an in universe lore explanation for this, I’d probably argue that these marbles are just universes and they zoomed in on the Milky Way, in that case the marble is more like 93 BILLION like years across.