I know that there's a fininite number of grains of sand on earth. I have trouble fathoming "infinity" and when I look at the universe I can't see it not having a wall at the furthest reaches of it. Kinda like the universe being in a big box. But if there is a box then what's on the other side of the wall? Just "nothing", just empty black space where nothing exists?
Like I said, it's too hard to comprehend.....
I believe the Universe is technically defined as the region where space and time exists. Space and time are continually being created as the universe grows/expands (now at an ever and ever faster, accelerating rate).