The observable universe, the universe that we can see is about 90 billion light years across, Which means that light would take about 90 billion years to reach from one point to the other. In this gigantic universe there are about 100 billion galaxies. Each galaxy contains between 100-1000 billion stars. Also we have recently understood that planets are very common .
Realistically speaking it is pointless to speak about any place in the universe other than our local group , a group consisting of our milky way galaxy and some clusters of galaxies. I say this because it is nearly impossible to travel beyond our milky way in search of extraterrestrial life as it would take billions and billions of years to reach there. So let us just focus on the milky way.
There are roughly about 400 billion stars in our home galaxy , the milky way. About 20 billion of them are sun like stars, a 5th of them contains earth like planets(habitable for life as we know it). Even if we realistically consider only 0.1% of these planets to have sustained life, it gives us about a million planets where life could have been formed.
Our milky way is about 13 billion years old. It is understandably very difficult for life giving planets to be formed in the early stages of a young galaxy, as everything is so explosive. However it would only take about 2 billion years from its formation for the milky way to create habitable stable planets. The only life sustaining planet that we know of “THE EARTH” is only 4 billion years old.
Now a civilization can be of 3 types:
A civilization capable of harnessing all sources of energy that its planet has to offer .if you want to know we are a 0.73 type civilization and on our way to become a type 1 civilization.
A civilization capable of harnessing the energy of its host star.
A civilization capable of harnessing and colonizing its entire galaxy.
If an intelligent life form could hypothetically build a spaceship that could sustain life for a thousand years it would take them a couple million years to colonize the entire milky way. Sounds like a lot of time? Remember the milky way had about 9-10 billion years , that is about 10,000 million years to do this
SO WHERE ARE ALL THE ALIENS??????
This is the Fermi paradox.
Given that there was so much time for intelligent life to develop, we have yet to find a definitive proof of its existence . No messages have been received , no messages have been answered , the universe seems to be a cold , lifeless place.So what is the conclusion??
Maybe it is very difficult for complex life to develop and we are just an anomaly. Maybe there is a galactic filter that wipes out all the intelligent life forms when it reaches a certain threshold .Or maybe we haven’t looked in the right direction yet…….toodles