The percentage of space discovered depends entirely on how “space” is defined. The cosmos operates on scales that defy human intuition, meaning our level of discovery varies dramatically between our local galactic neighborhood and the vast, distant reaches of the universe. To gauge our progress, we must distinguish between the physical space we have touched and the immense volume we have only observed through light. The percentage of the universe we have charted remains profoundly small.
Defining the Boundaries of Discovery
A meaningful percentage of discovery can only be calculated for a finite volume, which immediately excludes the total universe. Current cosmological models suggest the universe beyond what we can see is likely infinite, meaning any finite amount discovered represents effectively zero percent. Therefore, the concept of discovery is limited to the observable universe, the sphere of space from which light has had time to reach Earth since the Big Bang.
This observable boundary is a sphere approximately 92 billion light-years in diameter, defined by the distance light has traveled and the subsequent expansion of space. Everything outside this sphere is fundamentally unknowable to us because its light will never reach our instruments. Discovery within this boundary is further constrained by the horizon of the cosmic microwave background, which represents the earliest light we can detect.
Mapping Our Local Neighborhood
The only portion of space we have physically interacted with is our immediate local neighborhood, the Solar System. Unmanned probes have flown by every major planet and landed on objects from Mars to distant asteroids, but the volume of space they occupy remains insignificant. Even within our home system, vast regions of the Oort cloud, which is thought to contain trillions of icy bodies, remain entirely unexamined.
Expanding outward to the Milky Way Galaxy, which holds between 100 and 400 billion stars, the level of discovery drops precipitously. The European Space Agency’s Gaia mission has cataloged the precise positions and motions of nearly two billion stars, a monumental achievement in celestial cartography. However, this impressive number represents less than one percent of the total stellar population in our galaxy, illustrating the challenge of comprehensive mapping.
The star systems closest to us are the best understood. Astronomers estimate they have cataloged about 92 percent of the stellar objects within a region of about 336 light-years from the Sun. Beyond this small volume, dense dust clouds obscure the galactic center, and the sheer distance makes detailed study of individual stars in the outer arms impossible with current technology.
Cataloging the Observable Universe
Moving beyond our galaxy, the observable universe contains an estimated two trillion galaxies, each an island of stars, gas, and dust. The process of “discovering” this immense volume involves large-scale surveys that map the location and distance of these galactic structures. This effort focuses on charting the cosmic web, the vast network of galaxy clusters and voids that defines the universe’s structure.
Projects like the Sloan Digital Sky Survey (SDSS) have imaged over 35 percent of the sky, creating a three-dimensional map that charts the positions of millions of galaxies. These surveys have been instrumental in measuring the expansion rate of the universe and understanding its history over billions of years. Other deep-field observations, such as those from the Hubble Space Telescope, reveal tiny patches of the sky, showing thousands of previously unseen galaxies in a single view.
Despite these efforts, the volume of space thoroughly surveyed remains exceptionally small. The sheer number of galaxies means that even the most ambitious new missions, such as the Euclid space telescope, are only beginning the process of large-scale 3D mapping. Euclid’s goal is to catalog a third of the entire night sky, but initial project reports suggest that the full mapping effort is still only about one percent complete.
The Ultimate Unknowns of Cosmic Composition
Even if we were to map the location of every object within the observable universe, our understanding of its true nature would still be limited. The largest gap in our discovery is not the location of matter, but its identity. Everything we can see and interact with—stars, planets, gas, and dust—is composed of ordinary matter, which accounts for only five percent of the total mass and energy of the universe.
The remaining 95 percent of the cosmos consists of two mysterious, non-luminous components that we cannot directly detect. Dark matter makes up about 27 percent of the universe, inferred only by its gravitational influence on visible matter, such as the rotation of galaxies and the clustering of galaxy groups. Dark energy constitutes the largest fraction, approximately 68 percent, and is the unknown force driving the accelerated expansion of the universe.
Our current “discovery” is therefore based on observing the effects of these unknown components rather than understanding their substance. We have mapped the distribution of matter, but we lack fundamental knowledge about what most of that matter and energy actually is. Until we identify the physical nature of both dark matter and dark energy, our discovery of the universe remains restricted to a small, ordinary fraction of its true composition.