I'll take a stab at this one.
It'* expanding over areas of nothingness. Areas of void that are the most dangerous reaches of space. Think of it as an area that is so much a vacuum it causes everything in it to implode or otherwise be destroyed. As stars, planets, and other universe objects create gases and other matter, it pushes the boundaries outwards over the void.
Human kind will never see the edge of the universe. The technology it would require would be Star Wars or Star Trek material. Our race simply will not last long enough to develop that kind of technology.
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