If the universe is expanding, what is on the other side?

As far as we know, the Universe is infinite and has always been infinite, even as far back as the Big Bang.

One way to think about this is to imagine the Universe's density rather than its size. At the time of the Big Bang, the Universe was both infinite in size and infinitely dense. As space expanded, the Universe got less and less dense as each point in space moved away from every other point in space.

Note that while the Universe is infinite, we can never see all of it. Since it takes time for light to travel, we can only see the light that has been traveling for at most 14 billion years (the age of the Universe). We call this spherical region the observable universe. We think that the areas of the Universe that we cannot see are the same as the areas that we can see, but we have no way of knowing with one hundred precent certainty.

There is no "other side" in the conventional sense because the universe is expanding uniformly in all directions; rather, expansion is occurring everywhere, with galaxies moving apart as space expands as a whole.