Final answer:
Galaxies do not expand as the universe does; they retain their size as space stretches. The stars within a galaxy remain a consistent distance from each other because of gravity, and the cosmological redshift provides evidence for the expansion of space itself, not the galaxies.
Step-by-step explanation:
When discussing the expansion of the universe, it's important to clarify that galaxies themselves do not expand. Galaxies, including the stars within them, are bound by gravity and therefore maintain their size and internal structure as space expands. The expansion refers to the stretching of space itself, so while the distances between galaxies increase, the galaxies do not get bigger, and individual stars within a galaxy will not move away from each other.
This cosmological concept was supported by observations that all galaxies contain old stars, leading to the understanding that galaxies were formed some time after the Big Bang rather than expanding from smaller sizes. In addition, the cosmological redshift of light from distant galaxies serves as evidence for the stretching of space, as the increased distance causes light to shift to lower energies or redder wavelengths, without implying that galaxies themselves are growing in size.