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"?