Singular instead of plural noun in context

Question

Here is the context:

During the year after he published the general theory of relativity, Einstein applied it on the grandest of scales: the entire cosmos. You might think this a staggering task, but the art of theoretical physics lies in simplifying the horrendously complex so as to preserve essential physical features while making the theoretical analysis tractable.

Is this context talking about a specific "horrendously complex"? If not, why the author haven't used the plural noun, "complexes"?

[The original context]

Answer 1

Here, “the complex” means “what is complex”, “things that are complex”. It is the adjective complex used as a noun and does not relate to the meaning of the noun complex. This is not a very common construction.

To give other examples of the same construction, cosmology is often described as the study of the infinitely large while quantum physics is the study of the infinitely small.

Answer 2

"The horrendously complex" is another way or saying "something that is horrendously complex" so if you replaced it in the sentence you would get "... but the art of theoretical physics lies in simplifying something that is horrendously complex so as to preserve ...".

Then it becomes clear why this is a singular not a plural. It's a strange contraction since the noun ("something") has been left out while the "horrendously complex" is left. "Horrendously complex" might be an adjective, though my English grammar is not up to figuring our what part of speech it is, I'm afraid.