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NASA has assumed the universe is 13.8 billion years old.
And try to work within that scope. Read the following.
No single astronomical image reshaped our understanding of the universe like the Hubble Deep Field observations. Source: NASA Science https://share.google/HXNvK5nBJe6tFPs5h
Harry, there are innumerable "observed universes".
What is the total of these?
What about other collections of items including galaxies?
Maybe none are observed?
Then, the total of observed universes is "The Universe"? We do not know.
What if only one star is not observed?
So "The Universe" cannot be defined in any certain (unequivocal) way.