Re: Confused of Rochdale
Because the "dark matter" and more importantly "dark energy" is observed to be inside and just outside of galaxies.
It's "dark" in that we cannot see it, but we see the results just as we see the leaves blowing in the wind. It took centuries before someone actually "saw" what nitrogen, oxygen and carbon looked like.
In a similar way, all the galaxies spin at a speed and form in a way that suggests more matter and energy is somewhere in something we have not yet "seen". Though we see the stars and dust clouds spinning and drifting out to these "structures" or attractors. (In either halos of unexplained stars/dust around galaxies, or when tracking other observations).
It's not outside the universe, it's right here around us... somewhere. :P
I am not an astrophysicist or astronomer, but the above is how I read and have seen their observations. :)