That's actually dark energy, which while it shares a similar name, is fundamentally different. It is thought that dark energy is what causes space to expand. Not just the particles in space are moving away from each other, but the space in between them is actually expanding. This fact, discovered much in part by Hubble (you may recall the Hubble Telescope) is what lead to the Big Bang theory.
"In the real world, this would be a problem. But in mathematics, we can just define a place where this problem doesn't exist. So we'll go ahead and do that now..."