The definition according to many sources is: Anything that occupies space and has mass. So, the only thing that does not contain matter is vacuum as it has no mass (even though it occupies space). Perfect vacuum is a philosophical concept and can never be achieved in practice ( In the same reference, it is also told that perfect vacuum does not occur even in interstellar space.

As a result we can safely assume that each and every part of the universe contains matter, however small. That is, in real analysis, such a state is called “dense” — matter is “dense” in the universe just as the rational numbers are dense in the reals. That was just an aside.

When asked “What is Matter?”, the definition above might do for purely pedantic reasons but it does not provide the real answer. The definition above provides an answer to the question “What are the properties of Matter?” but not to the much more deeper question “What is Matter?”. May be this question has more to do with philosophy than with physics.