My opinion here:
Statistically speaking, it`s impossible for us to be the only intelligent life in this universe or for that matter, galaxy.
Too many stars and the possibility of planets that are capable of supporting life as we know it. The reason why we`re not visiting them is the distances involved. Would take at least 80 years one way to the nearest planet that is believed to be capable of sustaining us and as for SETI not finding extraterrestrial transmissions, there are way too many channels out there to be monitored with today`s technology. We`d be listening on one channel but missing a transmission on another.
As for advance races not communicating with us, perhaps they have what is known in Star Trek as `The Prime Directive`. Not interfering with the development of civilizations that are not as advanced. We certainly have a long way to go before we`re capable of interacting with other species.
Also, quoting from an old joke, the fact that aliens are not actively trying to communicate with us is proof that intelligent life exists!