If this question was asked to an astrophysicist or an astronomer, who knows the universe better than we do. He says, he doesn’t know.
“There are two terrible possibilities exist, either we are alone in the Universe or we are not.”
Perhaps, Life on earth is an accident that happened about 3.8 billion years ago. No one knows whether it came from some other part of the universe from some astroid or comet or it just began on in this planet. But it is a beautiful creation of nature.
May be it came from some distant galaxy or from a distant star of our own galaxy. This is called 'theory of panspermia.' No one knows. But, the widely accepted theory is the ‘primordial soup theory'. And it fits well enough to explain most of the questions on our existence.
Let us consider a star with one of its planets bearing life. After the death of the star and its planets i.e, after its explosion, its debris will be thrown into space. And if one of the pieces of it contains life, and if it hits another planet, and if the conditions on that planet is suitable for life to sustain, then the colonization of that life form takes place and later evolution will make the life form to fit well enough for their survival. May be this is the way how we came here, to this planet. This theory is called theory of panspermia. If this is true, aliens do exist and not only on one planet but on many. May be it just spreads like a virus, like a chain reaction from one planet to another. It still doesn’t answer the basic question, ‘how did life begin?' So, we go for another theory origin of species by primordial soup. It explains that, life did not come from outer space but it originated right here on this very planet. Until now this is the most accepted theory. And another theory says, all these happened 3.8 billion years ago.
We, perhaps, are here due to that accident that took place about 3.8 billion years ago. Gravity started doing something, atoms supported it and evolution took it to the next level.
So, you can’t get a definite answer to this question. After all what you can do is just guessing. No one knows who we are, why we are, how we are, where we are in this vast cosmic ocean.
To sum up in a sentence, “They might or might not.”
If there is life on this planet it's definitive that life exist in other forms and other planets too. We are carbon based life forms and have a spectrum of light thats percievable. There can be many more elemental forms of life or spectrum which we cannot percieve as of now. So, acceptance of other forms of life on other planets in this undefined Universe is a great way to percieve life!
They might exist and they might not. The galaxies are too big and we are yet to develop capabilities to explore life in far off galaxies.
However, I don't believe in the anecdotes of people seeing aliens on earth sometime in the past.
If Aliens are intended as green men in UFO culture I'm against their existence: I relegate them to mere conspiracy theory.
I believe in intelligent life outside of our planet, although we have no evidence in favor: I like believes that it is possible. SETI is searching for them :D
Since the only life form we know is born here on Earth it's not a easy task to define how an intelligent life can evolve.
I really like the theory of Great Filter (https://en.wikipedia.org/wiki/Great_Filter), you can find a great video here by Kurzgesagt: https://www.youtube.com/watch?v=UjtOGPJ0URM
I believe there are aliens out there, but I don't believe in the stories of alien abductions or them visiting earth on UFOs.
At least, they haven't landed on earth yet. The current pace of information technology and social media should have shown evidence by now.
If there are really aliens, they they must be smart enough arrive only on those times when cctv and smartphones don't exist yet.