Would it explain the acceleration of galaxies moving away from each other?
On a small scale, the universe would behave like a normal cube, so any matter would bend the space to some extent causing gravity. But, on a large scale, the convex universe would be pushing things away from each other and the farther you go, the more convex it would be.
A simpler example to understand this would be to consider the planet Earth. When we consider a small area, say 10 square meters, we perceive it to be flat and, in earlier times, that could have been one of the main reasons to believe that the Earth was flat. But, with some techniques and better understanding we now believe that the Earth is spherical (geoid) in shape. When we look at it from a larger perspective (not just the few square meters), we perceive the Earth as a sphere.
Similarly, when we look at the universe on a smaller scale (say, few thousands of light years), we perceive it to be flat... but, it may well be bent as a convex.
Now, back to pondering whether the universe is finite in space (having a boundary) or is infinite (unbound).