Years ago, back in the 1940's, 50's and even 60's the majority of the people in the world believed that aliens did not exist.  Today, according to many polls taken, some 52% of the population believe that aliens do exist and have visited this planet.  In short, the majority now believes that aliens do exist.  However, and this is a big HOWEVER, there is no solid proof of their existence.  At least not proof that can be scientifically proven to the general population.  

UFOs have been reportedly seen since the beginning of time.  In point of fact there are drawings on cave walls in South America depicting space ships and astronauts.  

Many believe our governments, not just the United States but Canada, Great Britain, Russia, and others, have kept it secret about aliens for various reasons, regardless of their value.  Again this is not something one can prove.  But the circumstantial evidence indicates that these governmental groups are keeping things secret from the general population for whatever illogical reason.  I say illogical because if the majority of the people on this planet believe in aliens there isn't much point in keeping anything about them secret unless you are up to no good in the first place...which is probably the reality of our governments.

Hope this answers your questions!