The story goes, when pondering about the life in the Universe Fermi asked: “Where is everyone?”
Given the vastness of the universe, even with a very small probability, there should be several intelligent civilisations. Where are they and why have we not found them? This is referred to as Fermi’s Paradox.
The explanation that is often provided is called the Great Filter. Each filter requires overcoming a tough challenge and life needs to pass through all those filters to reach a point where it can discover another.
For life to move from non-living to living form;
For life to them evolve from single-celled to multi-cellular;
For life to then develop intelligence;
For life to find a way to move beyond its own planet;
For life to colonise other planets;
Each step is an incredible challenge and not every life-form is expected to pass through all of these filters to the next stage. It is posited that these are the filters that have kept other highly evolved beings from emerging and being found.