Larry Sewell, age 12, ; for his question:

What exactly is matter?

The widest aspect of nature is the cosmos that includes the universe and everything in it. This infinite realm is governed by nature and we perceive it in many ways. However, everything we perceive can be divided into two separate groups    matter and energy. Energy is invisible force, shapeless and unbounded by space. It reveals itself to us in light, gravity, heat and many other aspects. Matter is of another order. It occupies space or volume. Even a submicroscopic particle occupies a set quota of space. It also carries a quota of gravitational force. As a rule, this gravitational attraction gives a particle of matter mass or weight when in range of other matter.

Scientists of the 20th century discovered that matter and energy are not so different from each other as they appear to be. In radioactivity, fragments of matter are converted into energy and under certain conditions energy can be converted into matter. The two great aspects of nature seem to be two sides of the same coin. Matter seems to be merely a frozen form of energy.