The observable universe has about 300 sextillion (300 followed by 21 zeroes) stars and about 200 billion galaxies. It is, therefore, estimated that there are about a septillion (1 followed by 24 zeroes) planets. As a comparison, the total number of sand grains on earth is 1 followed by 19 zeroes.
If some day we can prove the concept of multiverse as per the string theory, we can then say that there are universes beyond our universe. We can then multiply the above numbers several times over.
The earth has existed for nearly 4.5 billion years. If we consider this as a 24 hour clock, humans appeared in the last one second.
No matter how great the numbers of planets in the universe, the probability, therefore, that there is intelligent life existing somewhere else at the same time is not very high.
So, my very big doubt is - Are we alone? Whether it is so or whether it is not so, this very big doubt brings me to my biggest doubt - Who are we?