Aether

The Aether is a mini-biome that contains a cave made of Stone Block, a large pool of Shimmer in the center,  and Gem Trees. It also has a unique feature which causes all fully dark blocks to appear as a background with stars, resembling the background of Space. The Aether is located underground on the Jungle side's outer fifth of the world, usually below the Ocean. It can be found in the Cavern layer anywhere from 100 to 2000 blocks down depending on the world size. It's most commonly found a little bit above the layer where lava begins to appear.

Tips
It is recommended to construct a bridge made of Platforms over the pool of shimmer to make going from one side of the cave to the other easier, as well as prevent most Enemies from becoming invisible. This will cause Item Transmutation to be less convenient, so you should also make a small hole in the bridge.