10 Theories Why We Haven't Discovered Aliens Yet
6. They Are Too Far
A universe is a big place, so big in fact, that it got infinitely larger in the time that it took you to read that sentence. In comparison, the current rate of space travel is not going to get future astronauts anywhere significant. Now, the fastest confirmed particle in existence is a photon, a light particle. With our current understanding of physics, nothing can move faster than light. While that means the Millennium Falcon will have to wait, it means that space travel is a snail's pace.
While light speed would more than likely kill you in minutes from radiation poisoning, that would be the wall for space travel. Assuming humans ever managed to achieve light speed, and devise a way to not die immediately, that would be top speed for navigating the universe forever. The closest star to Earth (not our sun) is Proxima Centauri, about 4.22 light years away. It would take over four years to find the nearest star to Earth, much less search the outer reaches of the cosmos. Unless scientists discover and harness tachyons (theoretical particles that move faster than light), it is possible aliens are too far to reach.