When physicists say that the universe has no center, they are referring to the concept of the universe's expansion and its large-scale structure. According to our current understanding of the universe, there is no preferred or special point that can be considered the center of the universe.
The idea that the universe has no center is a consequence of the Big Bang theory and the observed isotropy and homogeneity of the universe on large scales. The Big Bang theory suggests that the universe originated from an extremely dense and hot state around 13.8 billion years ago and has been expanding ever since. However, it is important to note that the expansion of the universe does not mean that galaxies are moving away from a central point, as if it were an explosion with a center. Instead, the expansion of space itself means that the distances between galaxies are increasing on average.
The observations of the Cosmic Microwave Background (CMB), the afterglow of the Big Bang, have shown that the universe appears to be isotropic and homogeneous on large scales. This means that, on average, the universe looks the same in all directions, and there are no special locations that can be identified as the center.
It is crucial to understand that the concept of "center" is not well-defined in the context of the universe as a whole. The distribution of matter and the structure of the universe are complex and intricate, with galaxies forming clusters, superclusters, filaments, and voids on a vast cosmic scale. While galaxies may cluster together, giving the appearance of structures, these structures are spread throughout the universe without a single central point.
In summary, the absence of a center in the universe means that there is no privileged or preferred location from which the expansion or structure of the universe can be defined. The universe, as far as our current knowledge suggests, does not possess a specific object or point at its center.