Superstructures
While de Sitter space provides a model for an expanding universe, the question of the "largest possible space" in reality has a few different answers, depending on whether we're talking about what we can observe or what we theorize exists.
The Largest Observed Structures: Superstructures like Quipu
If we define "largest" by the most massive, coherent structures astronomers have actually detected, the current title-holder is a superstructure named Quipu. Discovered in 2025 by a team led by the Max Planck Institute, Quipu is a cosmic filament—a long, thread-like structure that is part of the universe's vast web [citation:2][citation:10]. It contains 68 galaxy clusters and has a mind-boggling mass equivalent to about 200 quadrillion suns [citation:2][citation:5]. Its length stretches for approximately 1.3 to 1.4 billion light-years, making it the largest known structure to be reliably characterized [citation:5][citation:10]. These superstructures are so massive that they actually affect cosmological measurements, like the expansion rate of the universe (the Hubble constant) and the cosmic microwave background [citation:5][citation:8].
A Potential Rival: The Hercules-Corona Borealis Great Wall
There is another structure that may be significantly larger than Quipu, but its existence is more debated. The Hercules-Corona Borealis Great Wall is a vast concentration of galaxies, mapped by detecting gamma-ray bursts (immense explosions from dying stars). Recent analysis suggests this structure could be an astonishing 15 billion light-years across [citation:1][citation:4]. If confirmed, it would be nearly 11 times larger than Quipu. However, because its detection relies on a less direct method, some scientists are more cautious about confirming it as a definitive structure [citation:1].
The Ultimate Limit: The Observable Universe
These enormous structures, as vast as they are, exist within a much larger sphere: the observable universe. This is not a physical object but a horizon—the maximum volume of space from which light has had time to reach us since the Big Bang [citation:3][citation:9]. Because the universe is expanding, the distance to this edge is not simply the age of the universe (13.8 billion years) times the speed of light. Instead, the current diameter of the observable universe is estimated to be about 93 billion light-years [citation:3][citation:6]. This is the absolute limit of our vision; we cannot, even in principle, see anything beyond this spherical boundary [citation:3].
The "Largest Possible Space": The Entire Universe
Finally, we arrive at the concept of the entire, unobservable universe. This is the truest answer to your question of the "largest possible space." The observable universe is just the tiny fraction we can see from our vantage point. According to the theory of cosmic inflation, the entire universe is staggeringly larger. Some estimates, based on simple inflationary models, suggest the whole universe could be at least 1.5 × 10³⁴ light-years across—that's 3 followed by 23 zeros times larger than the part we can see [citation:3]. Crucially, even this immense figure is a minimum estimate. The universe may very well be infinite in extent [citation:3][citation:6].
In summary, the largest known discrete structures are around 1.4 billion light-years wide, the observable universe is 93 billion light-years wide, and the entire universe may be infinite. This is what makes the question of a "largest possible space" so fascinating—it may not exist at all.