According to the theory of general relativity, gravity is a curvature of space-time caused by objects with mass. Because the universe is so massive, it actually curves back in upon itself, so there isn't an "edge" in the traditional sense. If you could fly a spaceship in a straight line forever, you'd eventually come back to your starting point, similar to an ocean liner or aircraft circumnavigating the Earth.
Pretty confusing, right? I took two semesters of university-level astronomy/astrophysics, and I still can't wrap my mind around some of this stuff. :/
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