The question of whether a galaxy or the Universe is bigger addresses the mind-boggling scale of the cosmos. To answer this, we must first establish what a galaxy is, examine how galaxies assemble into larger structures, and then clearly define the concept of the Universe itself. This provides the necessary framework for understanding the incredible difference in scale between a single star system and the entirety of existence.
Defining a Galaxy
A galaxy is an immense, gravitationally bound system composed of stars, stellar remnants, interstellar gas, dust, and a significant amount of dark matter. These structures are the primary building blocks of the observable cosmos, separated by vast stretches of near-vacuum. A large galaxy, such as our own Milky Way, often spans between 100,000 and 200,000 light-years across its main disk.
The Milky Way, a barred spiral galaxy, contains an estimated 100 to 400 billion stars orbiting a common center of mass. Its visible components include a flattened disk, a central bulge of older stars, and a spherical halo. Most of the galaxy’s mass is dark matter, which exerts a gravitational influence on everything within the system. Galaxies are sorted into categories based on their visual shape, including spiral, elliptical, and irregular.
Cosmic Structures Beyond the Galaxy
Galaxies cluster together, forming increasingly larger structures. The smallest of these gatherings are Galaxy Groups, which typically contain fewer than 50 galaxies and are bound together by gravity, such as our own Local Group. This Local Group, which includes the Milky Way and the Andromeda galaxy, spans a diameter of roughly 3 to 6 million light-years.
Galaxy Clusters are larger, more massive collections containing hundreds to thousands of galaxies. These clusters are the largest structures confirmed to be held together by gravity, preventing their dispersal despite the expansion of space. Beyond these clusters are Superclusters, which are enormous chains and sheets of galaxy clusters and groups that can span hundreds of millions of light-years.
The Laniakea Supercluster, home to the Milky Way, stretches over 500 million light-years and contains tens of thousands of galaxies. Superclusters are generally not gravitationally bound, meaning their component parts are slowly being pulled apart by the expansion of the Universe. These structures form a vast, web-like pattern known as the cosmic web, with dense filaments surrounding immense, mostly empty regions called voids.
Defining the Universe
The term “Universe” refers to the totality of all space, time, matter, and energy that exists. Cosmologists distinguish between the entire Universe and the Observable Universe, which is defined by our physical limits of sight. The Observable Universe is a spherical region centered on Earth, representing everything whose light has had enough time to reach us since the Big Bang.
Because the Universe is approximately 13.8 billion years old, one might expect this region to be 27.6 billion light-years across. However, the continuous and accelerating expansion of space means that distant objects have moved farther away since their light began its journey. Accounting for this expansion, the Observable Universe has a diameter of about 93 billion light-years.
This 93-billion-light-year boundary does not represent the edge of existence, but the limit of what we can physically observe. The total Universe extends far beyond this observable sphere, and cosmological models suggest it could be vastly larger, perhaps even infinite in size. In this context, the Observable Universe is merely a finite “bubble” within a much larger, potentially boundless reality.
The Definitive Answer and Scale Comparison
The Universe is larger than any single galaxy, as a galaxy is merely a structural component within the cosmic hierarchy. The Universe is the container for everything. The Milky Way, with a diameter of about 100,000 light-years, is dwarfed by the smallest measurable fraction of the Universe.
The Observable Universe is 93 billion light-years across. If a galaxy were metaphorically the size of a single grain of sand, the Observable Universe would be equivalent to all the sand found on a beach. The total Universe extends far beyond this observable region, making the comparison between a galaxy and the Universe an almost meaningless juxtaposition of scales.