String theory. One analogy that helps me is that if you have an elastic string with 3 markings on it, and you extend it, 1 and 2 would move apart, say, x distance in unit time. 2 and 3 would also move apart by x distance. Now, 1 and 3, however, move apart by 2x units in that unit time.

Since this speed of expansion keeps stacking up, you will reach a point (from your position) that is expanding away from you at the speed of light. Any light from beyond that point will never reach you because it's going away faster than light from there can come back.

And since we discovered that the rat expansion of the universe is increasing, a theory states that it would be impossible to see anything outside of our local group after a loong looong time, as slowly all the stars in the night sky start disappearing.