Universe
The Universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy.
Galaxy
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.The word galaxy is derived from the Greek galaxias literally “milky”, a reference to the Milky Way.
Astronomy
Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry in an effort to explain the origin of those objects and phenomena and their evolution