What number is not represented in Roman numerals?

The ancient Greeks were aware of the concept of zero but didn't think of it as a number. Aristotle had dismissed it because you couldn't divide by zero and get a down-to-earth result. The Romans never used their numerals for arithmetic, thus avoiding the need to keep a column empty with a zero symbol. Addition and subtraction were done instead on an abacus or counting frame. About 1,500 years ago in India, a symbol was used to represent an abacus column with nothing in it. At first, this was just a dot; later it became the '0' we know today.

Source: The Guardian