WHERE ARE THEY? (FERMI'S PARADOX)

This is the paradox named after Enrico Fermi, which questions the existence of extraterrestrial life or aliens.

It was the result of a casual conversation that Fermi had with his colleague at lunch where they discussed the UFO siting, which later shifted to the existence of extraterrestrial life and Fermi exclaimed: "where are they?".

The paradox is based on lack of evidence and probability which favors the existence of intelligent life being common in the universe, it also includes Drake's equation which is an attempt to find the probability of extraterrestrial life.

DRAKE EQUATION
(L can be the lifetime of the civilization/the star)

There are an estimated 2-4 billion stars in the milky way galaxy & 7 sextillion stars in the observable universe it's 7 with 22 zeros behind it, so the universe is pretty big and according to the math that there are some stars that are the size of the sun and some of them have planets which are in the goldy-lock zone where water can be in its liquid states, probability shows that there were probably between 1000 and 100,000,000 civilization in our milky way galaxy.

Since an evolved civilization might end up due to exhausting the resources on the planet the variable L is added in the equation to compensate that.

But Frank Tipper & John d. Barrow speculated that the average number of civilizations in a galaxy is much less than one.

Now considering the fact that there are infinite galaxies in the universe and the probability of civilization to the ability to travel interstitial similar to earth's technology it would take 5 million to 50 million years to colonize the galaxy.

in-spite of these facts we don't see aliens traveling across the galaxy and knocking our doorbell, there is the various explanation behind this one of them which is very famous is UFO conspiracy theory which states that unidentified flying objects are the spacecraft of aliens and they visit here and It has been suggested that the detection of an extraterrestrial radio signal or technology could well be the most highly secret information that exists.