The Universe almost certainly extends beyond the domain we can observe. If indeed the speed of light is finite and if our Universe has been around for only a fixed amount of time, we can access only a finite region of space—no matter how much technology might advance. We can see only those regions that can be reached via a light ray—or something else that travels at light speed—during the lifetime of the Universe. Only from those regions can a signal possibly reach us within the time that the Universe has been around. Anything farther away—beyond what physicists call the cosmic horizon
