When astronomers say that the universe is expanding, they mean that the distances between galaxies are increasing over time because space itself is stretching. This expansion means that galaxies are moving away from each other, and the farther away a galaxy is, the faster it appears to be receding from us. This is not because galaxies are moving through space away from a central point, but because space itself is expanding everywhere uniformly, so every observer sees other galaxies moving away from them. The universe has no center, and this expansion has been ongoing since the Big Bang about 13.8 billion years ago

. A useful analogy is imagining the universe as a loaf of raisin bread dough rising: as the dough expands, the raisins (galaxies) move farther apart, but they are not moving through the dough itself; rather, the dough (space) is expanding and carrying the raisins with it

. This expansion is described mathematically by the scale factor in cosmology, which increases over time, indicating that the average separation between objects like galaxies grows. Observations such as Hubble's law show that the recessional velocity of galaxies is proportional to their distance, providing strong evidence for this expansion

. In summary, the best description is that the universe is expanding because space itself is stretching, causing galaxies to move away from each other everywhere, not because they are moving through space from a central point.