In article , jacobnavia
writes:

Yes, I know about statistics, but how can a galaxy in only 750My grow
from zero to TWICE the Milky Way???

There are LIMITS imposed on the growth of galaxies by physical laws that
make galaxies of just 750My and that enormous size IMPOSSIBLE you see?

[[Mod. note -- Physical laws do indeed place limits on how fast galaxies
can form and grow. If we had solid and well-validated theoretical models
of galaxy formation/growth we could probably compute those limits.

But I don't think we have this level of understanding (yet).

So how do we know that 750 My isn't enough time?

Indeed. AFAIK, the IMF is not derivable from first principles; it has
to be put in "by hand". And this is something pretty basic.