The likelihood chances of an intelligent alien life form existing outside of our planet exists but it is very, very, very miniscule. Lets take a look at the scale of the Universe from the start of the Big Bang.
The universe is only 13.8 billion years old. Scientists know that this is a very young age of the universe and it can sustain itself till about 200 billion years when the universe will start decaying in vacuum of space.
Humans have only walked for about 250,000 years, as intelligent species developing tools etc.. This is less than a fraction of the time compared to the age of Earth let alone the age of the universe. This matters because we know that life is common in the universe according to the Drake’s equation; however intelligent life has very, very minimal chances for every habitable planet. For every galaxy with 400 billion planets, there will be about 10-18 billion planets capable of sustaining life, of which 20 million will be like earthlike exoplanets. However most of these planets have yet to produce environment that is sustainable of inhabiting life since it took earth about 1 billion years after its formation to create single cell organisms. We would need to factor age, atmosphere, and conditions to sustain life on the planet.
Then we would need to factor in time for life on that planet to develop properly from single-cell to multi cellular organisms which would take another 500 millionâ€“2 billion years for it to have multiple cells. Then, we would need to factor another 1-2 billion years of life to evolve from simple to multi-cellular organism to complex organism capable of surviving on its own in the wild such as land animals. Finally, we would need additional 400 million 1.6 billion years in order for life to show up as intelligent beings.
Earth is about 4.5 billion years old, it started to sustain life about 3 billion years ago. There is a certain pattern and phase of time where life will continue to evolve if the conditions on the planet allow it to unless there is a collision with an asteroid or explosions from its own environment or nearby stars or conditions become extreme. Intelligent species would need to watch out for itself from a catastrophe such as war, famine, virus, and other barriers to technology for reaching a type 1-3 civilization.
We have only started to look at other planets and sent out signals for only 50 years. That means light has only traveled 50 years for anyone located within that region will know that we exist which is highly unlikely for any advanced civilization that exists there to know we exist since our galaxy alone is 100,000 light years across. Radio signals have only travelled about 116 light years. With radio signals our communications can only travel so far within the cosmic distances of other planets. This is a severe technological barrier of limitation of our and alien civilizations.
Personally, I think life is very common within our universe; even for sure life exists in other planets within our galaxy right now as microbial life form. However a very few, maybe 2-5 digit Intelligent civilization also exist as of right now throughout our own galaxy but they are too far apart and too rare for any of us to communicate with each other and for light to travel within theirs and our lifetime to know that we coexist thus known as the great filter. I m sure that if we can find a way to send a signal out for our existence within 10,000 light years within our lifetime, an advance civilization would know we exist and possibly vice versa within our lifetime.
Our universe is constantly expanding and our observable universe is only 46 billion light years in radius. Not to mention the fact that the light we see from other galaxies, planets, and solar systems are from the past hundreds, thousands, millions and billions of light years in the past. We would see how the planet was formed or unable to see anything because it is showing us microbial lifeform. If aliens do exist today, we simply would not know of there existence currently and vice versa for them because the distances is soo vast because it takes such a long time for radio and light signals to travel. Even if we do see aliens with our telescopes, chances our their civilization has ended because it has taken light to reach from thousands to millions to billions of years to us and them.