The question of whether galaxies orbit the center of the universe is intuitive, drawing on familiar models like planets orbiting a star. However, the structure and dynamics of the cosmos defy this simple comparison. Galactic motion is not dictated by a single, central point of gravity, but by a combination of local gravitational attraction and the stretching of space itself. Understanding galactic movement requires letting go of the idea of a fixed cosmic center and embracing the geometry of an expanding cosmos. Scientists have developed models that explain how galaxies move relative to one another, showing that their motion varies dramatically depending on the scale being observed.
Addressing the Center: Why the Universe is Centerless
The universe fundamentally lacks a single center toward which all objects would orbit. This concept is formalized by the Cosmological Principle, which states that the universe is both homogeneous and isotropic across immense distances. Homogeneity means matter is distributed evenly throughout space. Isotropy means the universe looks the same in every direction we observe from our location.
This principle implies that our location is not special or privileged; every observer in every galaxy sees the same general arrangement of the cosmos. If the universe possessed a central point, observers closer to it would see a different structure, violating homogeneity. Therefore, the universe has no identifiable center because every point within it can be considered the center of its own observable region.
Many people mistakenly picture the Big Bang as a singular explosion from a specific spot. However, the Big Bang occurred everywhere simultaneously throughout space. As space expanded, it carried matter with it, meaning the “origin point” is not a location we can point to. It is more accurate to think of the Big Bang as the rapid expansion of space itself, rather than an explosion of matter into pre-existing space.
An analogy often used is the surface of an unbaked raisin bread loaf expanding as it bakes. Every raisin sees every other raisin moving away from it, and no single raisin is at the center of the loaf. This model illustrates how the expansion is uniform and how every point appears to be the center of the expansion from its local perspective.
Local Galactic Movement: The Role of Gravity
While the universe lacks a center, galaxies are subject to the powerful, localized influence of gravity. On scales up to a few million light-years, gravity overcomes cosmic forces to bind galaxies into cohesive structures. Galaxies, including the Milky Way, are constantly in motion, orbiting the collective center of mass of their local groups and clusters.
The Milky Way orbits the supermassive black hole at its center, Sagittarius A, just as the Sun and stars do. The Milky Way is a member of the Local Group, a collection of over 50 galaxies, including Andromeda (M31) and Triangulum (M33). These galaxies are gravitationally bound, performing complex, three-dimensional dances around the group’s shared center of mass.
The Andromeda Galaxy is currently approaching the Milky Way at approximately 110 kilometers per second, exhibiting a blueshift rather than the typical redshift. This motion contradicts the general expansion of the universe, demonstrating that local gravitational attraction is strong enough to pull these massive objects toward each other. The two galaxies are predicted to collide and merge in about 4.5 billion years, forming a new, larger elliptical galaxy often nicknamed “Milkomeda.”
Local gravitational dynamics are profoundly shaped by Dark Matter, an unseen substance making up about 27% of the universe’s total mass-energy content. Dark matter provides the gravitational scaffolding that holds galaxies, clusters, and superclusters together. The orbits and movements of galaxies are largely determined by the distribution of this invisible mass.
The True Large-Scale Motion: Cosmic Expansion
On the largest observable scales, galactic movement is not an orbit around a center or simple motion through space. The defining motion of the cosmos is the expansion of space itself, first observed by Edwin Hubble in the late 1920s. Hubble established that galaxies are generally moving away from one another, and the farther away a galaxy is, the faster it recedes.
This relationship is known as Hubble’s Law, providing the primary evidence for an expanding universe. The recession velocity is not due to a physical push; instead, the space between galaxy clusters is stretching, increasing the distance over time. The light from distant galaxies is stretched along with the space, causing its wavelength to increase, a phenomenon known as cosmological redshift.
Thinking back to the raisin bread analogy, the raisins (galaxies) are not crawling away from each other through the dough; rather, the dough (space) is rising and expanding, carrying the raisins farther apart. Within gravitationally bound systems like the Local Group, local gravity is stronger than the expansion of space. However, on the vast intergalactic scales between galaxy clusters, the stretching of space dominates, causing the clusters to move apart.
The expansion of the universe is not constant; it is accelerating, a discovery that surprised scientists in the late 1990s. This acceleration is attributed to Dark Energy, a mysterious repulsive force estimated to constitute about 68% of the universe’s total mass-energy. Dark energy acts as an “anti-gravity,” pushing space apart at an ever-increasing rate.
The existence of dark energy means that receding galaxies will continue to accelerate their motion away from our Local Group. In the distant future, the expansion may become so rapid that galaxies beyond our local gravitational influence will move away faster than the speed of light, leaving them beyond observation. This large-scale, accelerating motion of space is the definitive answer to how galaxies move on the cosmic scale.