Many believe it to be a myth, while some say they have proof
It is basically whatever you believe...
We are one solar system revolving around 1 star. There are a definite of 100-200 billion stars in our galaxy only. There are many other proven galaxies around the world, they have their own stars, so just think about the possibility??
And aliens, don't necessarily have to be animals, they can be bacteria, protists, fungi, plants anything..!! They just have to qualify for a living characteristic and be found outside the Earth's atmosphere, to be considered "alien".