I don’t know.
Probably a variety of things, some ordinary, others maybe defying explanation. Use your imagination: Frozen turds dropped by passing intergalactic spaceships?
They’re receiving renewed attention because of an impending Pentagon report, which by all accounts won’t say we’ve been visited by aliens, and recent media coverage set off by disclosure of several Navy videos our government says are authentic, but which are open to interpretation.
That background is explained in a Vox article published on June 18, 2021, which you can read here.
Now for my own conjecture.
It’s hard to wrap the human mind around the immensity of the universe, even just what we can see or detect with our instruments. Our own galaxy, the Milky Way, is thought to contain somewhere around 100 – 400 billion stars; and astronomers believe there are at least two trillion galaxies.
That’s a lot of stars, and the odds no planet but ours harbors intelligent life are mathematically an incalculable fraction of an undetectable speck this side of zero. And when you figure the universe has been around for ~13.8 billion years, it’s not very plausible there’s never been a technological civilization anywhere except here. But they’re probably not very common, which means they’re probably spaced pretty far apart, and it’s unlikely they all exist at the same time.
The problem, as I see it, is that the separation in space and time is too great for us to establish contact with aliens, or them with us.
As far as we know, the physical laws of the universe are the same everywhere, and unchanging with time. By those laws, the speed of light is a limits how fast anything can travel through space. So unless there’s some shortcut we don’t know about, a hypothetical alien civilization in the nearest galaxy to ours would need 25,000 years to send us a signal, and another 25,000 years to receive our reply. By the way, that’s a pretty small galaxy, with lower odds of harboring a technologically advanced civilization.
Any alien visitors (or probes) most likely would come from our own galaxy. (Andromeda, the nearest big galaxy to our own, is 2.5 million light years away; if someone there is looking at Earth right now, they’re seeing it before humans existed.) Our city lights and radio emissions have been going out into space for over a century, and there are at least a few hundred stars and probably some rocky planets within that radius. But that doesn’t mean they’ve ever had intelligent life, or if they did, it’s still around.
It’s possible aliens in more distant regions of our galaxy have sent out probes. Like thousands of years ago, because that’s how long it would take for them to get here. But why would they bother, if they must wait thousands of years to get back signals? Moreover, if we’re being visited by aliens, why do they only check out military installations, ships, and planes, and not our cities? That’s more likely to be another country than alien visitors. Or electronic or visual illusions that civilians don’t see because they don’t have the equipment that creates those illusions.
I think aliens probably exist somewhere, but I’m skeptical we have extraterrestrial visitors. There’s no firm evidence we do.
For what it’s worth, I once saw a “UFO.” It was a yellow light in the sky, far away, at high altitude, making right-angle turns and traveling at high speed. Or at least appeared to be. This was in an area near Mt. Rainier famous for UFO sightings. But the fact I don’t know what it was doesn’t make it an alien spacecraft. What I thought was a distant flying object might have been my optical nerve acting up, or a lightning bug a few feet from my nose. Perception, even by radar and cameras, can go haywire. Sometimes things we thought we saw might not exist at all.
Related article: What scientists think (read it here). Spoiler alert: (A) They’re not aliens; (B) government investigators are incompetent.
Related stories: Could telescopes detect city lights on other planets? Read article here. If UFOs are alien spaceships, they’re probably probes, not crewed vehicles. Read that opinion here.
By being unidentified flying objects that means we don’t know what they are. That brings up security issues particularly when thousands of people see them over Washington on a nice summer day. This caused serious concerns in the Truman and Eisenhower administrations. There have been other widely viewed and reported events. The US government has used combinations of the noble lie, nothing to worry about, covering up classified projects rather than the truth of we have not a clue.
For some of these sightings using Occam’s razor extra terrestrial craft is a plausible explanation, even more so than a secret NAZI base hidden in Antarctica spying upon a world and preparing NAZI supermen and women for a springtime for cloned Hitler and Antarctica. Space is very large yet some of the things in sci fi are at least plausible. Bending space frankly brings anywhere in the universe a mere step through a passage way away. Warp drive involves a cheat that does allow faster than light travel. It maybe impossible to detect faster than light travel unless you can do better than ftl.
Technological advanced civilizations should actually be common in the Milky Way Galaxy, and planets with life extremely common, many may not be a place you would want to live or could live.
Our solar system was just visited by our first recorded interstellar visitor. Oumuamua just sailed through the milky way, and it has some aspects that it could be a really big spaceship, but not a classic space ship with Buck Rodgers in suspended animation or a really thin long rock with a mystery formation.
In quantum entanglement two atoms will match each other no matter the difference between them, making instantaneous communications possible across hundreds, thousands or billions of kilometers or lightyears.
Bob Lazar of Area 51 fame is likely a liar and perhaps Uncle Sam was playing with a particle accelerator. Still others are just reporting what they saw or recorded. I rather doubt Big Foot exists. UFO’s and Big Foot is at least possible in the Star Wars universe, fun to think it could be in ours. I am not a nerf herder.
Some people believe anything. I think you’re one of them.