The statement that the universe has no center is based on our current understanding of the large-scale structure of the universe and the observations made by astronomers. However, it is important to clarify a few concepts related to the expansion of the universe and its geometry.
When we say that the universe is expanding, it means that the distances between galaxies on average are increasing over time. This expansion is not like the expansion of a balloon's surface into pre-existing space. Instead, it is the expansion of space itself. It is crucial to note that this expansion does not require a center from which everything is moving away.
The analogy often used to describe the expansion of the universe is the raisin bread analogy. Imagine a loaf of raisin bread baking in an oven. As the bread dough rises, it carries the raisins with it, causing them to move apart from one another. In this analogy, each raisin sees all the other raisins moving away, but there is no central raisin from which all the others are receding.
Similarly, in the expanding universe, every observer, no matter where they are located, sees galaxies moving away from them. This observation is a consequence of the expansion of space itself. It is a property of the universe on large scales and does not imply the existence of a center from which the expansion originates.
The current best-supported model of the universe is the cosmological principle, which assumes that, on large scales, the universe is homogeneous (looks the same everywhere) and isotropic (looks the same in all directions). This model, combined with observations of the cosmic microwave background radiation and the large-scale distribution of galaxies, supports the idea that the universe has no center and is roughly the same everywhere on a large scale.
It's worth mentioning that the observable universe, which is the portion of the universe we can currently observe, is indeed flat according to our measurements. However, the overall shape and curvature of the entire universe beyond the observable universe are still subjects of ongoing scientific investigation and debate.
In summary, the statement that the universe has no center is a result of our understanding of the large-scale properties and observations of the universe. The expansion of the universe does not require a central point of origin, and our observations suggest a homogeneous and isotropic universe on large scales.