The region in an atom that contains most of the mass is called the nucleus.

Here's why:

* Protons and Neutrons: The nucleus is made up of protons and neutrons, which are much heavier than electrons.

* Electrons: Electrons, which orbit the nucleus, have a much smaller mass compared to protons and neutrons.

Therefore, the nucleus, containing the majority of the atom's mass, is the densest part of the atom.