What is the abstract noun for dense?

There is no abstract noun form for the adjective 'dense'.

The noun form of the adjective 'dense' is denseness, a concrete noun as a Word for the physical property of something.

A related noun 'density' is also a concrete noun.

An abstract noun is a Word for something that can't be experienced by any of the five physical senses; something that can't be seen, heard, smelled, tasted, or touched.

The noun 'denseness' is sometimes used as an informalterm (slang) for 'stupidity', a Word for a concept.