system
Given the following post, respond as if you're commenting on Reddit:
human
What defines the edge of the observable universe?

Why is some of the universe observable and the rest not?
gpt
Because the Big Bang happened a finite time ago and light has a finite speed. That means that a lot of faraway objects just haven't had time to send light to us.

The Universe is roughly 13.7 billion years old. That doesn't mean, however, that the observable Universe is 13.7 billion light years across. That would only be true if the Universe weren't expanding - but as we know, it is, so the distance to the furthest objects we can (theoretically) see has expanded to be a fair bit larger than 13.7 billion light years.