Oh, sure, we have all sorts of equations to dally with, so to wish and dream with a more realistic approach, such as the Drake equation
.N = R * f p n e f l f i f c L
However, even the most detailed scientific equation won't provide any proof of what might, or what might not, be out there.
Statistically speaking, extraterrestrial life should exist, yet, Fermi's Paradox seems to indicate that it doesn't.
Regardless of how bleak the outlook is, or seems to be, we can at least estimate how many intelligent civilizations should exist in our galaxy.
Using Drake's Equation, according to my estimates, there are no less than 1200 intelligent civilizations in our galaxy!
I used several factors to help determine the inputs to the equation, but you can use your own, if you want to try it. Drake himself, came up with 10,000 intelligent civilizations.
So does that mean there are between 1200 and 10,000 aliens roaming around in Earth's skies? No, of course not. As the key word here is intelligent civilizations, not star faring civilizations. But, assuming there are at least 1200 intelligent civilizations, then those that survive long enough to attain the stars would probably amount to less than 1%. Or, about 10. That is, 10 star faring civilizations!
Now, if we use Drake's number of 10,000 intelligent civilizations, and apply my guesstimate of less than 1%, we get around 90 star traveling civilizations.
Hence, the UFOs you see in the sky come from no less than 10 civilizations and no more than 90 civilizations.
But, I think my number might be too narrow and Drake's just a little broad, we should further refine the result. So...
Add the results together:
10 + 90 = 100
Then, find the median average:
100/2 = 50
Yes, that number seems to "feel" better.
There are probably 50 star travel capable civilizations crossing the seemingly endless space between the stars in the Milky Way galaxy, and probably even visiting our Earth.
So, why haven't we seen them? Well, I think we have. I even think certain governments of Earth have definitive proof of their existence.
Will we ever know for sure? Probably not.
But, we can wish and dream.