Other galaxies observed by astronomers are almost all redshifted. This means that most galaxies are moving away from our galaxy. Since this is true in every direction, the galaxies are all moving away from each other.

Question

Imagine that the universe is a solid ball with galaxies scattered all through it from the center to the edge. Suppose the galaxies stay the same size, but the ball gets bigger. What will happen to the distance between the galaxies?
The distances between all the galaxies will increase throughout the ball.
The distances between the galaxies near the center of the ball will decrease.
The distances between only the galaxies on the surface of the ball will increase.
The distances between the galaxies will change randomly depending on the galaxies' motion.

Answer 1

The correct interpretation of the expansion of the universe, analogous to your solid ball scenario, is that the distances between all the galaxies will increase throughout the ball.

As the ball gets bigger, the fabric of space itself is expanding, which causes the distances between all galaxies, regardless of their position in the ball, to increase. This expansion is uniform and affects galaxies at all locations, leading to an increase in distance from one another. The underlying concept is that the space between galaxies is stretching, which results in a redshift of the light from these galaxies as they move away from each other. So, the first option is the best representation of the situation.