As outlined in the starting post, this pack is inspired by Lemmings 3D above all else.
Remember how Lemmings 3D had levels on the Practice rank forcing you to use its new features? Like Highlight Lemming and Virtual Lemming?
There were even some levels in the main ranks that placed all cameras inside a restricted area - famously "The Prisoner" - so that you absolutely had to solve them in first person.
Well, with this important context in mind, so that you don't just consider this pointless trolling, I made the ultimate level to enforce / teach a new player to use clear-physics mode:
Pitch-black universeFirst of all, thanks a lot to Colorful Arty for enabling this madness! 
Clearly, he did his best to prevent such shenanigans by making the default background for his
Silhouette tileset (that is, if you don't use the orange one that comes with it) completely white.
Thus, this wasn't as easy as simply hitting "clear background" in the editor, because even if most other tilesets have a default black background, none of them actually include it as a graphic.
And in turn, if I had set the level's main theme to a different one than Silhouette, then the staircases, platforms, stacks etc. and, most of all, the lemmings themselves would have become visible again.
I was about to go out of my way to deliberately add a black background graphic to my strato_generalmd tileset (like everyone's special tileset they use for custom additions).
But then, I discovered this background from one of GigaLem's Freedom Planet sets, which fits the title even better. You do still see something, and indeed you see a universe... and yet, you see nothing.

Maybe the hatch, the exit, and the trap, but none of the terrain. Even the reflections of the water in the bottom left corner can just look like additional stars in this screenshot. Of course, once you start the level, you will see the water moving.
Fortunately, Arty's Silhouette tileset doesn't include triggered traps. The one thing I confused for a masher trap turned out to be a teleporter (and I was wondering why it didn't do anything... well, of course, I hadn't placed any receiver in the level, because I didn't want any teleporters in it in the first place). But there is a completely black version of the Marble slasher fire trap, so I used that one instead.
Hidden traps are bad because they can surprise you.
But hidden lemmings and completely hidden terrain

- well, that just forces you straight into clear-physics mode, so that no nasty surprises remain.