What is the function of 的 in the sentence below? 在有形的以太

It should mean "here in tangible ether" but I don't understand the function of 的. 

Can't I say the exact same thing writing it like that: 在有形以太 
?
Between an adjective and a noun, you can add a 的 if you like.

美麗的姑娘 beautiful girl
好玩的游戲 entertaining game
複雜的語法 complicated grammar

Adding the 的 is the norm if the adjective has two characters. But it's possible to leave it out:

黑暗森林 dark forest
意外事故 unexpected accident
美好時光 happy time

Why did the poet use 的 here? Some possibilities:
- 以太 is an unusual word, putting a 的 before it makes the sentence easier to understand.
- The poet may have liked the flow of the sentence better with 的.
- Using 的 here is more normal than not using it.