Are aliens real? We don't know for sure, but we want to believe.
Outer space is a vast expanse that we have so much more to learn about, which is why it's hard to flat-out deny the possibility that other intelligent lifeforms exist. New species are continually being discovered in the ocean, and some animals thought to be long gone have emerged from the thick cover of jungles and the deep sea to be rediscovered.
If life can exist—and persist—in seclusion and in some of the harshest conditions on Earth (just look at tardigrades), it's likely that other interplanetary lifeforms have evolved and acclimated to conditions in space, too.
The renowned science writer Arthur C. Clarke once said, "Two possibilities exist: Either we are alone in the universe or we are not. Both are equally terrifying." Several discoveries and theories from some of the greatest minds in science point to the likelihood that there is something beyond us in the universe, so there's a pretty decent chance we have neighbors somewhere in the ether. Consider the evidence.
In 2007, the Department of Defense (DoD) created a program called the Advanced Aerospace Threat Identification Program (AATIP) to study "space-related phenomena that could not be easily explained, usually involving the appearance of high-speed, unidentified aircraft," per New York magazine's Intelligencer.
The covert program was headed by military intelligence official Luis Elizondo, who sought to investigate reports of UFO encounters. A decade later, Elizondo quit working at the Pentagon and confirmed AATIP's existence to the New York Times.
Avi Loeb, an impressively credentialed scientist who taught at Harvard and chaired the university's Astronomy Department, has put forth an interesting, but seemingly far-fetched, hypothesis: the asteroid Oumuamua is actually space debris from an alien structure or a defunct alien space craft.
Coming from anyone else, this might seem zany. Again, Loeb knows a thing or two about the machinations of space. However, while truthers are soaking the Oumuamua theory up, Loeb's colleagues are highly disappointed and upset that he's posited what they're calling an "insult [to] honest scientific inquiry."