Fermi paradox
Extraterrestrial life might exist, but humans may not know about it. To send signals across long distances in our galaxy, and the universe, is difficult. This is because the power output needed to project the signal would be very large. Coherent (laser-like) signals travel at the speed of light, but our own galaxy is 100,000 light years across. Alien civilizations may not be able to detect the signal, or they may not understand the communication or the method used to communicate.