Why is the observable universe limited even though space may be much larger?

The observable universe is defined by the farthest light that has reached Earth since the Big Bang. Although the universe is about 13.8 billion years old, expansion means the most distant visible regions are now much farther away than 13.8 billion light-years. We see only a portion because light from regions beyond our cosmic horizon has not had enough time to arrive. This is why the observable universe is about 93 billion light-years across, not 27.6 billion. The rest of the universe may exist, but it is permanently unobservable to us if it lies beyond our horizon.