Where is almost all the mass of an atom?

Answer: At the nucleus

Explanation:

The nucleus of an atom has very small dimensions. However, it occupies its central part and concentrates more than 99% of its total mass.

It is in the nucleus that the protons (positive charge) and neutrons (neutral charge) are found (having both almost exactly the same size). This is why when we talk about the atomic mass of an atom we can say it may nearly reperesent the measure of the size of the nucleus of an especific atom.