Here's why:
* Abstract nouns refer to concepts, qualities, or ideas that cannot be touched or seen. "Vastness" is a concept related to size or extent.
* "Vast" is an adjective that describes something as being large in size or extent. When you use "vast" as a noun, you are referring to the abstract idea of largeness itself.
For example:
* Adjective: "The desert is vast." (Describing the desert's size)
* Noun: "The vastness of the universe is awe-inspiring." (Referring to the concept of vastness itself)