Vibe: Thanks for reminding me. I do remember reading about those. I believe the open system would be shaped something like a saddle; a hyperbole, I think.
Chimp: There is also dark energy in addition to dark matter, theorectically. Gravity holds things together at the local frame of reference for galaxies (and also between this in clusters of galaxies), however, some type of force stretches across the entire Universe to pull it apart.
As the theory goes now, dark energy comprises 65% of the Universe, dark matter is 30% and actual matter is a measly 5%.
While on the subject of Universe expansion there is also the two ideas of either galaxies becoming frozen in time, or, that time never ends.
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