When we look up at the night sky, it appears densely packed with countless stars, suggesting a universe brimming with substance. This visual experience is deceptive compared to the reality of cosmic scale. The intuition that space is full comes from observing the dense, local neighborhood of stars within our own galaxy. Yet, even within the Milky Way, physical matter occupies a negligible fraction of the total volume. To understand how much of the universe is truly empty space, we must analyze the sheer volume separating celestial objects. This analysis reveals that the overwhelming majority of the cosmos is not filled with stars or planets, but is instead characterized by profound voids.
The Scale of Cosmic Distance
The concept of emptiness in space is defined by the massive separation between gravitationally bound structures. Within a single galaxy, the space between stars is referred to as interstellar space. Even in the dense spiral arms of the Milky Way, the average distance between stars is measured in light-years.
For instance, the nearest star system, Alpha Centauri, is approximately 4.3 light-years away. To visualize this scale, if the Sun were shrunk to the size of a tennis ball, the nearest star would be located hundreds of miles away. This demonstrates that even our stellar neighborhood is overwhelmingly composed of empty volume, with matter highly concentrated into tiny, isolated pockets.
The true vastness of the cosmos is revealed in the intergalactic medium, the space between entire galaxies. The distance between major galaxies is measured not in light-years but in millions of light-years, or megaparsecs. For example, the Andromeda galaxy resides approximately 2.5 million light-years away from the Milky Way.
This intergalactic volume represents the largest source of cosmic emptiness. Galaxies are like tiny islands of matter floating in an ocean of void, where the average density of material is exponentially lower than anything found within a star system. This immense separation characterizes the universe as fundamentally empty of visible structure.
Defining the Vacuum
While interstellar and intergalactic space appear empty, a scientific definition of a perfect vacuum reveals a more nuanced reality. A true vacuum—containing absolutely no particles, energy, or fields—does not strictly exist anywhere in the universe. Even the most desolate regions of the cosmos are permeated by a residual background of particles and energy.
The intergalactic medium, while incredibly sparse, contains an average density of about one to ten atoms (mostly hydrogen and helium) per cubic meter. This density is many orders of magnitude lower than the best vacuums we can artificially create in laboratories on Earth. This medium is also traversed by cosmic rays, neutrinos, and the faint, pervasive electromagnetic radiation known as the Cosmic Microwave Background.
Beyond sparse matter, the vacuum of space is a dynamic environment defined by quantum field theory. This theory suggests that space is constantly bubbling with “virtual particles” that briefly pop into and out of existence, a phenomenon known as the quantum vacuum. The presence of these fleeting particles means that the physical geometry of space itself is never truly void of activity or potential energy.
Therefore, when astronomers discuss “empty space,” they are primarily referring to regions devoid of significant concentrations of ordinary, baryonic matter. The physical space is not a barren nothingness; it is a low-density medium existing far from the gravitational influence of large structures.
The Universe’s Hidden Majority
The previous discussion focused on the emptiness of space relative to visible, baryonic matter, which is merely the tip of the cosmic iceberg. All the stars, planets, and gas clouds that we can observe make up a small fraction of the universe’s total mass-energy density. This visible matter accounts for only about 4.9% of the total composition.
The remaining 95% of the universe is filled with two mysterious, invisible components: Dark Matter and Dark Energy. Dark Matter accounts for approximately 26.8% of the total mass-energy budget. It is defined by its gravitational influence on visible matter, such as how galaxies rotate faster than expected based on their visible mass.
Dark Matter is thought to be non-baryonic, meaning it is not made of protons and neutrons, and it does not interact with light or electromagnetic forces. This substance is believed to form a vast, invisible scaffold or halo within which galaxies and clusters are embedded. Its pervasive distribution means that even the regions between galaxies are laced with this unseen mass.
The largest component of the cosmos is Dark Energy, making up about 68.3% of the total mass-energy. Unlike Dark Matter, which clumps gravitationally, Dark Energy is uniformly distributed throughout space. Its defining characteristic is that it acts as a repulsive force, driving the accelerated expansion of the universe.
The presence of Dark Energy intrinsically links it to the volume of space itself. As the universe expands, the amount of Dark Energy increases proportionally, maintaining a near-constant density. This makes the vast regions between galaxies the primary reservoirs of this dominant energy component, fundamentally altering the definition of cosmic emptiness.
The Final Composition
Synthesizing the concepts of distance, low-density medium, and hidden components provides a quantified answer to the question of cosmic emptiness. The universe’s composition is broken down into 4.9% ordinary matter, 26.8% Dark Matter, and 68.3% Dark Energy. This composition dictates the nature of the space we inhabit.
If “empty space” is defined as the volume devoid of the 4.9% of visible, baryonic matter, then the overwhelming majority of the cosmos is empty of stars and planets. The massive distances between galaxies ensure that nearly all the volume of the universe is a low-density void relative to our everyday intuition.
However, if “empty space” is defined as the volume devoid of all mass and energy, the answer changes entirely. The pervasive presence of Dark Matter and Dark Energy means that the universe is completely full, not with visible substance, but with a complex mixture of unseen energy and mass.
The final paradox is that while the universe is almost entirely empty of the material we can see, it is simultaneously saturated with the components we cannot. The space itself is filled with the energy driving its expansion and the mass dictating its structure, making a true, perfect void an impossibility.