marvel - How are the Nine Realms situated in space?

So both before the Marvel Cinematic Universe and up until Thor: The Dark World, I always thought the Nine Realms were more akin to parallel dimensions than anything else.

However, in The Dark World,

during the final battle, Mjollnir gets lost and keeps having to change direction as Thor and Malekith fall through portals between realms. At one point, the hammer makes a right angle and starts heading out into space, fairly definitively showing that at least that one realm is accessible solely through space travel.¹

So if they are all accessible through space, what constitutes a realm? Why only nine, given the expanse of the Marvel universe? Is it because those planets are near each other, or situated on some cosmic Yggdrasil, or something else entirely?

_{¹The Bifrost itself never really qualified to me as proof, because of its magical/technological nature. The directionality could just be a requirement to access a specific location within Asgard, like how Loki's secret passage was off in the distance.}