If space is a vaccuum and there is no gravity then why is the universe round and what holds it together?

Question: If space is a vaccuum and there is no gravity then why is the universe round and what holds it together?
- Keywords:
  gravity,
  universe,
  vacum
Asked by maahamsaleem to Rob on 22 Jun 2013.
- Robert Woolfson answered on 22 Jun 2013:
  
  So the universe isn’t actually round, according to some recent NASA measurements (and don’t ask me how they worked this out, I have no idea), the universe is actually flat. This shape is due to the relationship between the actual mass of our universe and something called the critical mass, which will eventually control the fate of the universe.
  
  http://map.gsfc.nasa.gov/universe/uni_shape.html
  
  What holds the universe together? That’s a bit more of a mystery. The current theory is that the universe is held together by something called Dark Matter. As the name suggests, we can’t really observe it at the moment and we aren’t really sure what it is. All we really know (to the best of my knowledge) is that there must be something more than we’ve currently seen to explain a lot of behaviour in astrophysics.
  
  http://www.guardian.co.uk/science/2013/may/26/dark-matter-jeff-forshaw

Question: If space is a vaccuum and there is no gravity then why is the universe round and what holds it together?