The concept of life on other planets dates as far back as ancient Greece. Epicurus proposed that the processes that created our world likely have created other worlds elsewhere - with their own plants and animals similar to, but unlike, Earth. This theory was opposed even then, as many believed that Earth was not only the center of the universe, but the only planet in existence.
Intelligent life and it's speculation doesn't stop at faraway planets in other galaxies. In fact, some believe it hits much closer to home. UFO terminology and cultural impact were largely popularized in the United States in the 1940s, but reports and stories of unexplained aerial phenomena have been ingrained in human history since over 1000BC.
Today, the question of UFOs is massively contentious; either completely discredited and ridiculed by the media and scientific circles, or sensationalized and fabricated for entertainment. Level-headed reporting is usually ignored in favour of clickbait. In today's age of disclosure and official statements from whistleblowers and law enforcement, it's important to approach everything with a critical mind and do your own research.