Bridge to learning: Space does not expand, like a fabric or something that can be stretched (a misconception introduced by most models). Likewise, the Universe does not expand "into" anything, and since there is no space "outside" it. Instead, the scale that is used to define space changes. For this reason, it may be helpful to talk about the spacetime of the Universe expanding rather than saying that space is expanding.

In this video from JPL of the Expansion of the Universe, a series of waves are sketched on an elastic band. Visualize that there are points on either end of the sketched waves. Imagine there is a term that defines the distance between those two points. As the band is stretched, the term remains the same, but its scale has changed.

Bridge to learning: Constructing a Hubble plot places our galaxy (and Earth) at the origin of the graph. This may be interpreted by students to mean that our galaxy occupies a unique position at the center of the Universe. It’s important to show that the same perspective would be seen by observers in other galaxies as well. This can be addressed by working through questions that use the galaxy scrambler interactive in this investigation. Having students view a Hubble plot based off the observations from other galaxies can be a powerful way to help them understand that all locations all see the same expansion away from their location, so therefore no location is the center of the Universe.

This website provides some other ways to engage students with examples of how we can deduct from observations that there is no center to the Universe.

Bridge to learning: Within a group of galaxies, the gravitational attraction of galaxies upon one another is strong enough to prevent the cluster from expanding, even as the spacetime of the Universe expands. Local gravitational attraction can cause galaxies in clusters to approach each other or even merge. As a result, some nearby galaxies have velocities that show that they are moving towards the Milky Way galaxy.

Bridge to learning: A steep slope for a Hubble plot indicates that expansion rate for the Universe is “fast,” whereas a galaxy with a fast recessional velocity (from the observer) is plotted toward the top of the graph. Have students compare the value of two points at very different distances from the observer to see that the galaxies’ recessional velocities are different and the slope of the graph at the two points (1) would be the same, if the graph is a straight line (constant expansion) or (2) different for the case when the graph is curved and indicating that the Universe’s expansion is accelerating.

Bridge to learning: The difference is that in the first case, galaxy velocities are due to their physical motions through a static volume of space, whereas cosmological redshift attributes the motions to the expansion of spacetime itself. Although each galaxy does have a velocity, the cosmological redshift component of its motion far exceeds any minor contribution from a galaxy’s velocity, with the exception of galaxies very close to the Milky Way Galaxy.

Use one of the common models (inflating a balloon, the raisin bread (or chocolate chip cookie) analogy, stretching a rubber band) to demonstrate the expansion of space.

Bridge to learning: The expansion of spacetime is only evident when observing galaxies at large distances, beyond the Local Group of galaxies. Within the Solar System or a cluster of galaxies, mutual gravitational influences keep objects gravitationally bound to each other.

Our observations of the redshifts of light from galaxies allows us to develop a model of how the Universe is expanding. Using this model, we can make predictions about how other locations in the Universe will observe expansion. Our observations tell us that the Universe is expanding uniformly. As space expands it causes galaxies to appear to move away from one another. No matter where you are in the Universe, you will observe galaxies moving away from you, with nearby galaxies moving away more slowly (since there is less spacetime expanding between them) than far away galaxies (which have a greater amount of spacetime between them). So, observers in all galaxies will make the same observations regarding the motion of galaxies and the expansion of the Universe.

Since all locations observe expansion away from them, no one location can be the central point for expansion, and therefore the Universe has no center.

No one knows why the Universe is expanding, but the theory of general relativity mathematically supports the role of gravity and its relationship to spacetime, predicting the result that we see. https://www.universetoday.com/116229/whats-causing-the-universe-to-expand/

This is still an unresolved question. Astronomers have given the name “dark energy” to describe whatever may be causing this acceleration, but no one really knows yet what it is. What we observe is that dark energy acts to oppose gravitational attraction on large scales and makes the entire Universe expand more quickly, and it seems to be uniform throughout the Universe. https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy