I don't think we have been visited by aliens but I wouldn't rule out that they exist somewhere. We haven't even ruled out other forms of life existing in our solar system yet on Mars, Europa or Titan.
There are around 100,000,000,000 stars in our galaxy, how many planets orbit those stars is anyones guess, but we are finding more and more of them all the time. We have found the first evidence of extra solar water [link=http://antwrp.gsfc.nasa.gov/apod/ap070417.html]http://antwrp.gsfc.nasa.gov/apod/ap070417.html[/link] and we have found rocky planets [link=http://www.universetoday.com/am/publish/large_rocky_planet.html.]http://www.universetoday.com/am/publish/large_rocky_planet.html.[/link]
A few other links
[link=http://www.users.muohio.edu/weaksjt/]http://www.users.muohio.edu/weaksjt/[/link]
[link=http://en.wikipedia.org/wiki/Extrasolar_planet]http://en.wikipedia.org/wiki/Extrasolar_planet[/link]
That's just in our galaxy.
There are at hundreds of billions of galaxies out there, maybe even upto 500 billion. Not all are the same as ours some have more stars some less, but they reckon there are about 70 sextillion stars in the universe, that's 70,000,000,000,000,000,000,000
http://www.cnn.com/2003/TECH/space/0...vey/index.html which is going to make for an even larger number of planets.
If you ask me there will be intelligent life out there somewhere.