But since the universe apparently has a finite age, est at about 13.7 billion years, what would have been the "original" size of the galaxies in that case? And where would they have come from?
So "they" say... But isn't time relative depending on the observer and what is being observed? "They" say time is fixed and finite. What about the astronauts who experience a lesser elapsed time than those they left behind on Earth? Two different elapsed ages, in the same elapsed interval that has passed. We're on Earth, distant receding galaxies are estimated to be so many light years away and that is extrapolated into them being a calculated age in terms of elapsed time. Because they are moving away, perhaps their observed elapsed time is not 13.7 billion years, but a much lesser interval, perhaps only millions of years. Or maybe trillions of years. Maybe scientists are getting it all wrong. Perhaps the universe is infinitely old, and the original size of galaxies was enormously larger than speculated. While still adhering to the Big Bang theory, where they became instantly enormous exploding from a single point. We don't know what we don't know, and I'm not losing sleep over it because it really doesn't matter.