answer:Emptiness is probably infinite, although we have no way of proving that other than by thought experiment. Spacetime is defined as distance between things. Nothingness is outside the concept of spacetime, but you are quite correct, there is no circle as shown in the two diagrams. That was probably just an unfortunate convention the artist used to group the galaxies together. The Universe could be infinite, but we can’t observe enough of it to definitively say. We do know it has a radius of at least 70 billion light years. We also think that while all the other galaxies seem to be rushing away from us, with the most distant one going fastest, leaving the impression we are at the center of the Universe, that it would actually appear that way no matter where you go in it. So either is is infinite or spacetime is non euclidean and has more than 3 spatial dimensions. The expansion of the universe is established by measuring what’s called the redshift of galaxies near to us and others further away. We find that the more distant a galaxy, the greater its redshift, therefore the greater its speed of expansion away from us. What is expanding is spacetime. It isn’t like everything is expanding equally. The Earth isn’t getting further away from the Sun because of expansion of the Universe. The stars of our Milky Way galaxy aren’t getting further from its center. But the spacetime between our galaxy and all other galaxies is growing. And the growth gives the appearance of the most distant objects from us rushing away from us at greater than the speed of light. Now, here is a thought to toast your noodle. If the universe is infinite, what is left outside of it for it to be expanding into?