Hi shrutikasat2703,
The issue of "countable" vs. "non-countable" is a minor grammar rule that you'll likely be tested on once (but possibly more than once) in the GMAT Verbal section.
"Countable" nouns (eg. chairs, people, books, etc.) can be referred to with the words "fewer" and "number" (among others).
"Non-countable" nouns (eg. air, sleepiness) can be referred to with the words "less" and "amount" (among others).
Kevin points out that you don't need an article in front of a non-countable noun, which is true (since matter is "non-countable", it falls into that category - you might refer to "less matter" or an "amount of matter").
GMAT assassins aren't born, they're made,
Rich