So I will post here since i will be avoiding spoilers.
However i will use a video of the game's longplay to support my statements since seeing it animated is way different from seeing images.
To begin with, the galaxy didn't become hollow, so i retract my concerns here.
Here's what i mean. There's something dark in the middle, something flame-like, but it is shifting and it is on the inside of the galaxy. This is NOT space. Yeah this isn't a hole right through the galaxy. My evidence is 1. the fact that it's very shifty and mobile and 2. I deadass went in to make sure they are not the same. I used a tool to look at which color is the hole itself and what color is the space in this instance. Space is #18182f while the inside of the galaxy is #1a1222. Essentially, while it's hard to make out due to similar levels of darkness, one is blue and the other is purple, so they are vastly different in reality.
Next up is the fact that the galaxy has become even more galaxy like unless i missed it before. I mean, they made it a lot more spiral-like, it now has more features similar to a galaxy. Look at it here, it clearly has more distinct spiral arms than it did before.
Next up we need to talk about the speed of spin.
It spinning in my opinion is not an issue. The gravitational pulls inside AD are crazy af I mean do you see the planets move around like crazy? It's clearly not your average dimension and nothing states it has to be the same as the normal universe in every way so in my opinion the galaxy spinning fast is normal and there's also the fact that, as i've mentioned before,
this is how galaxies in kirby's universe are. Ironically i'd say they look a bit less like galaxies than the galaxy in our image from AD at this point. I will of course say that this is probably a stylistic choice for this already cartooney game and they simply didn't want to change too much.
Finally, could it be a gravitational well? I doubt it. I mean this is a colored structure with defined spiral arms, stars inside and outside of it, a now defined center and the fact that it spins. I believe that it is enough to prove that this is a galaxy now.