Galaxies are vast collections of billions of stars, gas, dust, and unseen material, all held together by gravity. These immense cosmic structures are not static; they are in constant motion, and yes, galaxies do orbit things in the universe. The nature of this movement changes significantly depending on the scale of the structure being observed. The movement of entire galaxies is governed by a complex hierarchy of gravitational interactions, ranging from tight, localized paths to massive, directional flows across the cosmos.
The Force Driving Galactic Movement
The fundamental mechanism dictating the motion of galaxies is gravity, where the total mass of a system determines its influence on surrounding objects. This gravitational pull binds galaxies together and dictates their speed and trajectory through space. Surprisingly, the majority of the mass within and around galaxies is not visible matter, but an invisible substance called dark matter. Dark matter does not interact with light, but its gravitational effects are undeniable and profound.
Scientists infer the existence of dark matter because stars at the outer edges of galaxies orbit much faster than visible matter alone can explain. A substantial dark matter halo must be present, contributing approximately 85% of the total mass in the universe. This mass acts as the primary gravitational anchor, ensuring galaxies remain coherent structures. It governs their movements in groups and clusters with a much larger gravitational potential than visible matter suggests.
Galactic Neighbors and Direct Orbits
On the smallest organizational scale, galactic movement most closely resembles a classical orbit, with galaxies bound into gravitationally closed systems known as groups and clusters. Our own galaxy, the Milky Way, is a member of the Local Group, which contains over 50 galaxies. The two largest members are the Milky Way and the Andromeda Galaxy, currently separated by about 2.5 million light-years.
These two massive spiral galaxies are on a direct collision course, approaching each other at approximately 110 kilometers per second. They are orbiting a shared center of mass, known as the barycenter, which represents the gravitational balance point between them. This attraction will culminate in a merger event in about 4.5 billion years, forming a new, larger elliptical galaxy. This scenario exemplifies a gravitationally bound, closed orbit, where the mass density is high enough to overcome the overall expansion of the universe.
The distinction between a galaxy group and a galaxy cluster lies primarily in size and mass. A group, like the Local Group, is a modest collection of up to about 100 galaxies, dominated by a few large members. A galaxy cluster is a much larger structure, containing hundreds or thousands of galaxies, often with a large central elliptical galaxy and vast amounts of hot, X-ray-emitting gas. Both structures are gravitationally bound, with individual galaxies orbiting the collective center of mass.
Regional Flow within Superclusters
Moving up the cosmic hierarchy, groups and clusters are assembled into even larger structures called superclusters. These superclusters are the largest structures defined by a shared, directional movement, which is less of a tight orbit and more of a large-scale, gravitational flow. Our Local Group resides within the Laniakea Supercluster, a colossal structure containing about 100,000 galaxies and spanning over 500 million light-years.
The Laniakea Supercluster is not a single, gravitationally bound entity, but is defined as a “basin of attraction.” All member galaxies are flowing toward a common gravitational focal point directed toward a region known as the Great Attractor. The Great Attractor is a massive, diffuse concentration of matter composed of several large galaxy clusters. The Milky Way, along with its neighbors, is hurtling toward this region at a speed of around 600 kilometers per second relative to the cosmic background.
This regional flow is a direct consequence of the uneven distribution of mass in the universe. The Great Attractor exerts a powerful collective pull on everything in its vicinity, creating an immense stream of galaxies. It acts as the local gravitational minimum, drawing in all the matter in its basin. This movement is a transitional scale where the sheer size of the structure results in a shared, organized drift rather than a simple orbital path around a single center.
Motion on the Largest Scale
The largest known structures in the cosmos are superclusters and vast filaments of matter that string together, separated by enormous, nearly empty voids. This large-scale architecture is often described as the Cosmic Web. Galaxies and superclusters move along these strands, drawn toward the largest concentrations of mass.
However, when considering the universe as a whole, the concept of orbit breaks down. The universe is not orbiting anything, nor does it possess a central point of gravity. The ultimate, overarching motion of the cosmos is its expansion, where the space between gravitationally unbound objects is actively stretching. This expansion is governed by dark energy, an enigmatic force that counteracts gravity and drives distant galaxies further apart at an accelerating rate.
While gravity holds together galaxies and small clusters, it cannot overcome the expansion of space on the largest cosmic scales, such as the space between superclusters. Therefore, the movement of a galaxy is a combination of two distinct effects: a local gravitational orbit or flow toward nearby masses, and a passive movement away from distant galaxies due to the metric expansion of space. The local movements define galactic neighborhoods, but universal expansion defines the ultimate fate of the cosmos.