Fair warning: this question needs a philosophical mind, will require a "short" lecture to answer, and is a question you might come across in college. have fun!
if, where you are standing, is inside of a building, which is inside of a community, which is inside a city, which is inside a country, which is inside the world, which is out in space, and that space is always growing, then what is it growing in?
this means that if space is growing, where does it grow into? a void? a place of blankness? darkness?
we humans, apparently, like to have boundaries. we like to know our limits and how it fits to the world around it. but when we don't know our limits, such as the solar system back before the year 2000, we tend to be bothered to the point when we grow a bit insane.
and the day we realized the solar system had no solid boundaries, rather, fade out into space, it bothered us a little more, knowing the solar system had no solid boundaries.
so, to recap, we humans like boundaries. we like to know the end or limit of something. without that, it is really hard to truly understand the universe. and that can really bug us. so a question has been around for a while. if the universe is theoretically growing and expanding, like water spreading on a surface, what does it grow into?