system
Given the following post, respond as if you're commenting on Reddit:
human
Since the universe is expanding, how do we know if a large portion is not already 'out of sight' and thus put it at a much younger age than it might be?

Science noob here, just wondering.
gpt
As a matter of fact we know that a large portion IS already out of sight.

We can measure the current rate of expansion and use that to extrapolate the age of the universe. Also note that this is just one of several methods of estimating the age of the universe, and they all converge on a similar figure of around 13 billion years.

On another note, we have a definite minimum age of the universe which is based on the distance of the most distant visible thing. We know how long it's light would have taken to reach here, and we know how fast light travels, and so we can use that to give a lower bound on the age of the universe. That thing is only a few hundred million years younger than the estimated age of the universe (and when you consider how long it takes for complex stuff to form, that is quite reasonably consistent with the 13 (or so) billion year estimate.