In a very brief and incomplete response, no, we do not yet know if aliens exist or not.
What counts as an ‘alien’?
In my research, I am going to count only lifeforms with high enough developed technology level to send out signals to outerspace. This would mean that the extra-terrestrial lifeform is at our level of civilization or higher. Therefor, microbial life does not count.
What are some tools used to find aliens?
The Square Kilometre Array searches a whole kilometer of the sky in detail. Breakthrough Listen is a project in construction which would cover 10x the area. Exomars is a high profile mission searching for methane gas in the atmosphere which would signify life.
What are the chances that we will find aliens?
Science has proven that it’s more and more likely that extraterrestrial life exists. According to a model by Claudio Grimaldi, we’ll have to thoroughly search in a 40,000-mile radius of the sky to be fairly certain that there is no other alien species as advanced as us out there.
Why haven’t we found aliens yet?
Humans have not existed for long enough with enough technology to find proof of alien life, and alien civilizations have maybe risen and fallen before or after us. Another theory is there may be a barrier on a technological level before or after us that stops life from progressing further. If the barrier is behind us, all other aliens have died off before becoming as advanced as us, and if the barrier is ahead of us, we will soon reach the point where our species will die off. Finally, there can also be alien species that advanced in a totally different technological directions and is not traceable by signals.