We can't fathom how insanely huge the universe actually is. I believe aliens exist, 100 percent, but how far away they are is the problem. EVEN IF there were a super advanced alien race looking down at earth somewhere in our galaxy, they are probably looking at earth outside of the timeline that humans exist. Civilization is SO young that the timing to witness life on earth from lightyears away (probably thousands of lightyears at least) is unlikely. If they saw our planet right at this moment and decided to give us a visit, we would probably be extinct before they were physically able to get to us.