What is the difference between the universe and the galaxy?

In essence, the term "universe" refers to everything that exists, including galaxies and their intergalactic medium.

We live in a galaxy called the Milky Way, which is a vast collection of stars held together by gravity, numbering in the millions or billions.

The term "universe" referred to our galaxy before it was discovered that there were other galaxies.

For a while after it was discovered that there were other galaxies, the term "island universe" was used to refer to galaxies; however, it has become obsolete. In certain respects, it is still a fitting term because the vast distances involved would make traveling to another galaxy extremely difficult, impractical, or nearly impossible.

There are about 50 galaxies in the Local Group, mostly smaller satellite galaxies, with Andromeda being the largest galaxy visible with the naked eye. The Milky Way is the second largest galaxy, followed by the Triangulum Galaxy.

There are at least 100 groups of galaxies in the Virgo Supercluster, which includes the Local Group of galaxies.

It's possible that the Virgo Supercluster is a component of the roughly 100,000-galaxie Laniakea Supercluster.

The known universe is thought to contain roughly 100 billion galaxies.

All matter, energy, planets, stars, galaxies, and everything in between are collectively referred to as the universe. A galaxy is a massive system of stars, stellar remnants, interstellar gas, dust, and dark matter that is held together by gravity. Galaxies are distinct structures within the universe, but they are also contained within it.