A galaxy is one gravity-bound system of stars, gas and dust, such as the Milky Way, while the universe is everything that exists, containing billions of galaxies. Our solar system sits inside just one arm of the Milky Way galaxy, and that galaxy is only a tiny part of the universe, which scientists estimate is roughly 13.8 billion years old.
The core difference
A galaxy is one system of stars within the universe, which is everything that exists.
- galaxy — a massive system of stars, gas and dust held together by gravity: Telescopes have photographed galaxies billions of light-years away.
- universe — all of space, time, matter and energy that exists: No one knows exactly how large the universe really is.
How to tell them apart
The memory hook is the word galaxy itself: it comes from Greek galaxias, short for milky circle, describing the pale band of light the Milky Way makes across the night sky. A galaxy is a single one of these gravity-bound clusters, while the universe is the whole container that holds every galaxy there is.
The mix-up shows up when people use the words interchangeably to mean space in general. A useful check is scale: if the sentence could contain the word billions of times over, it probably means universe, not galaxy.
