system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] Are the 9 Realms all in the same dimension?

Could each realm be traveled to via spaceship without the use of the Bifrost? (albeit very slowly)

Are the 9 Realms, just 9 different planets, all in the same universe/galaxy? 

Does this mean planets like Xandar aren't in any of the 9 realms?
gpt
The 9 realms are 9 planets connected by a wormhole network called Yggdrasil. They were naturally connected through the convergence (Thor the Dark World) which made it easier for the Asgardians to build Yggdrasil and then take over/oversee them. Odin and the Asgardians are the 'space police' of the 9 realms, like the Nova Corps is for the others. Perhaps that's why Thanos was so reluctant to gather the stones prior to Odin's death ; he was afraid of him.