#class dismissed

For starters, I had originally assumed that the Prison Dimension the Krang were in and the Twilight Realm where the Shredder had been sealed was the same place, but after much consideration I have decided that is not the case.

The Twilight Realm for example, is very easy for Yokai to access, which is why Karai still had to seal Shredder away within that realm rather than just toss him in and leave him to rot like they did with the Krang. What's more, the ONLY way to access the The Prison Dimension is through the Key (or by literally tearing open the fabrics of existence as Mikey had).

So, let's look at the information we can gather from the movie about the Prison Dimension:

Gravity exists (though it varies and is much lighter than Earth's)

Air exists (otherwise Leo would have suffocated and sound would not have been able to travel)

Difference in Passage of Time (?) (otherwise Krang would have to live for a very very very long time, which they probably DO live much longer than humans, but I doubt they could live THAT long... though still long enough for most to die out.)

Death is possible in this realm (as stated above).

Stars or some sort of light source seem to exist (can easily see them in the BG at varying distances)

Physics (movement/speed/impact) seem to be impaired

This final detail is one I'm PARTICULARLY interested in. Did you ever notice just how... slow Leo and Krang Prime move in the Prison Dimension? As if they're moving underwater or through molasses. Krang Prime literally can move at mach speeds on Earth when it suits him, but in the Prison Dimension he can't even move fast enough to catch up to Mikey's portal in the climax of the film. It also might explain why Leo was able to take such a beating and still survive if Prime's attacks were being weakened by the environment. Sure he could have been pulling his punches but... he was so angry and having lived in that prison for centuries he is probably very aware of how the environment affects his attacks.

Last but not least we have Splinter's brief explanation of the dimension's origin, in which he states that the Key was a weapon FORGED by the mystic warriors of the past to lock the Krang in a prison dimension. Does this mean that they created the dimension as well? This might make sense if it's something that can ONLY be accessed by that Key and might also explain why Mikey could basically "pick the lock" since he is a Hamato as well.

So what can we gather from this information?

It's NOT deep space

It can NOT be accessed like other parallel dimensions

It's a place that seems to defy the law of physics

It seems to have a different passage of time

CURRENT WORKING THEORY: most likely a small pocket dimension created by the mystic warriors specifically designed to contain and keep the Krang weak.

While there are light sources all around the background we have no guarantee that they are actually stars. It could be sources of light from Krang tech being used to keep the place lit (which makes sense given their consistently pink hue).

What's more the distant pinpoints of light might simply be debris reflecting those light sources. It's hard to say but it makes sense since if there were other solar systems around then the Krang would have likely moved towards those other locations in search of resources.

Sadly this is only a working theory based off of details I have noticed. I would be very interested to hear other people's theories on what that place could possibly be. It's all very mysterious but I think that's what I like about it. It's allowed for some very unique interpretations in other AU's!