I am not sure there is one. Galaxies contain stars, gas, dust, black holes and other dead stars, and - above all - dark matter. The dark matter typically makes up 80-90% of a galaxy's mass.
Quartz is the mineral that is the basic building block in many rocks.
basic of spine
The major and basic building blocks of proteins are the amino acids.
the cell
It is the basic building block of life
The basic building block of matter is the atom.
Quartz is the mineral that is the basic building block in many rocks.
basic of spine
Amino acids are the basic building block to proteins.
An "Atom" is the basic building block of all molecules [first] and [then] biomolecules.
cells
the cell
An Atom. Molecules are two or more atoms chemically combined. Atoms are the basic building block of matter.
the basic block of the silicate minerals are formed from rocks.
Tissue
carbon
It could be argued that the most basic building block of the body, as in all other things, is the atom. The answer that you are most likely looking for is the protein.