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.
The answer depends on how "basic" you want to get. Stellar systems, stars and planets, molecules, elements, nucleons and electrons, quarks. Take your pick.
Quartz is the mineral that is the basic building block in many rocks.
basic of spine
Carbohydrates
Cells.
A cell is the basic building block of life.
The basic building block of matter is the atom.
atoms are the basic building block of matter
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
Carbohydrates
Cells.
An Atom. Molecules are two or more atoms chemically combined. Atoms are the basic building block of matter.
The basic building block of the body is cells. Cells come together to form tissues which then come together to form organs.