Do you believe in aliens and UFOs? What about places like Roswell, New Mexico?
Now for the big question : Is there intelligent life elsewhere in the universe ? Dunno, but the odds look good.
How can we know the odds? By looking at ourselves. We are critters that live on a planet and have managed to do so long enough to build what passes for a civilization. We can thus estimate the number of other civilizations in the universe by following the Drake equation [3, 4]:
N = G * S * P * h * l * i * c
Where N is the number of civilizations, G is the number of galaxies in the universe, S is the number of stars per galaxy, P is the number of planets per star, h is the fraction of those in the “habitable zone”, l is the fraction that go on to develop life, i is the fraction that develop intelligent life, and c is the fraction that go on to develop civilization. Of these factors, we have a good handle on the first two, and are getting some interesting information on the next two.
We know that there are about 500 billion galaxies in the universe (G=500,000,000,000). And we know that there are about 100 billion stars in a typical galaxy (S=100,000,000,000). Based on our observations of exoplanets, it appears that nearly every star will develop planets, so we can arbitrary put P at 0.9. Also based on our exoplanet work, we know that about 10% (h=.1) of those will be in the habitable zone; i.e., neither so close to their sun that the planet is too hot for life, nor so far out that it is too cold.
From there, we go into complete speculation. Some exobiologists think that life is inevitable; others think that it is exceedingly rare. Let’s go with the latter and assume that life happens in one out of every billion habitable systems (l=.000000001). Intelligent life is presumably even rarer , so let’s arbitrarily put that at one in a trillion (i=.000000000001). However, once a species in intelligent, civilization seems inevitable; both of the known intelligent species developed civilizations, so we can put c=1.
Putting it together, we get:
N = 500,000,000,000 * 100,000,000,000 * 0.9 * 0.1 * 0.000000001 * 0.000000000001 * 1 = 4.5
So there should be about three other civilizations out there . Using different values for intelligence and life give different results; reasonable values range from 2 to 20,000.
And that leads to the Fermi Paradox: “Hey! Where is everybody?” After all, if all of these civilizations existed, then there should be some evidence of them, right? Well, maybe not. You can argue (and many have) that the ETI  are there but space is so big that we haven’t had time to get their signals. You can argue (and many have) that they exist but will not communicate with us due to some “Prime Directive” . Or you can argue (and many have) that the act of communication is so fraught with assumptions that they are shouting at us, but we simply cannot understand them .
For my money, they are out there, but are waiting to see if we are grown up enough yet to be allowed to eat at the grown-ups table . As one wag noted, the surest sign that there is intelligent life in the universe is that it hasn’t tried to contact us.
 And the one that I ssupect you really meant to ask.
 Avoiding the much harder question of “Is there intelligent life on Earth?”
 Named after Frank Drake, who first codified it in 1961 while working at the Green Bank Observatory.
 There are actually several versions of the equation, which differ based on the method used to determine the number of stars. I give the simplest one.
 Think about it for a moment. Of all of the species on Earth, only two (possibly three) are known to have developed intelligence. We speculate a lot about others, but have no firm evidence for (or against) intelligence on their part.
 I have avoided the question of “when do these civilizations happen” by avoiding the rate of sellar formation. If that is included, then the number of civilizations goes up.
 Extra-Terrestrial Intelligence, as exosolar civilizations are known in the astrobiology lab. ET strictly means the External Tank of the Space Transport System (aka the Space Shuttle).
 I find that excuse a bit idiot-idic.
 Yes, I do know how To Serve Man. Why do you ask?