[raybbr]

They kept bouncing back and forth between the original 'center' of the Universe (where they hit each other and rebound) and the end of the Universe, which is hard like a turtle's shell.

Huh? It's all one big giant ball and we're on the inside? What happens when solar systems start crashing into the "wall"?

[UCANSEE2]

What happens when solar systems start crashing into the "wall"?

Well, first off it is whole galaxies, not just single solar systems.

Second, what happens is that they bounce off just like the radio waves, and come careening back at random angles toward the center of the Universe.

That explains why not all galaxies are moving AWAY from the center of the Universe.

[dr_lew]

All this reminds me of the second entry in the Oxford Book of Science Fiction Stories, Finis by Frank L. Pollack, written in 1906. ( H.G. Wells' The Land Ironclads is the first. )

In Finis an astronomer theorizes that there is a huge "central sun" to the universe, but its light hasn't gotten to the earth yet. Of course, it duly arrives and the story is occupied with describing the resultant cataclysm.