It is obviously the prison for an Elder One. The immense gravity is needed to keep them drowsy and not stirring too much.

Who need chains or tractor beams? A planet gravity is the perfect restraint one needs for these kinds of inmates. Low maintenance and high effectiveness. Let just hope they aren't getting any jailbreaking going on now that we have disturbed the quantum harmony of the prison by observing it...

Maybe we just know a lot less about the universe than we previously thought?

Likely an experiment by Puppeteers.