Fact and Speculation.
The detected material mass of the matter in the Universe is so small
(the average density of all substance in the Universe is approximately
p=10^-30 g/sm^3) that it cannot ‘close’ the Universe into sphere and
therefore our Universe as whole is ‘open’, endless Vacuum.
But what to do with the infinite Universe the physicists don't know.

Socratus, how do you explain p=10^-30 g/sm^3) to grandmother? smile
The Universe on a T-Shirt
E=MC2 + GOD--service-based as agape/love