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 cosmology, the universe does not have a specific "center" in the way that, for example, a circle has a center point.
To understand this, we need to consider the idea of the Big Bang, which is the prevailing theory of how the universe originated. The Big Bang suggests that the universe began from a hot, dense state approximately 13.8 billion years ago and has been expanding ever since. However, it is important to note that the Big Bang was not an explosion that occurred at a particular point in space. Instead, it was the expansion of space itself.
In this expanding universe, every point can be considered as moving away from every other point. Imagine the universe as the surface of an inflating balloon. As the balloon expands, every point on its surface moves away from every other point, but there is no specific point on the balloon that you can call the center.
Similarly, in our universe, as space expands, every location appears to recede from every other location. This expansion occurs on a large scale and is observed in the form of the redshift of light from distant galaxies. The absence of a center implies that there is no privileged location from which the expansion is emanating.
It's important to note that the concept of the universe having no center refers to the large-scale structure and expansion of the universe. It doesn't mean that galaxies or galaxy clusters cannot have their own centers. In fact, galaxies can have centers around which their stars and other celestial objects orbit, but these centers are not the "center" of the entire universe.
Our current understanding suggests that the universe is homogeneous and isotropic on large scales, meaning that it appears the same no matter where you are in it (on a large enough scale). This concept is known as the cosmological principle. However, on smaller scales, such as within galaxies and galaxy clusters, there can be local variations and structures.
In summary, when physicists say that the universe has no center, they mean that there is no specific point within the universe from which the expansion is originating. The universe is expanding uniformly, and every location in space can be considered as moving away from every other location, giving the impression that there is no central point.