They could be breeding us
“Aliens could do to us what we did to wolves, selectively breeding a once noble species in grotesque ways, transforming us into the equivalent of bulldogs, poodles, dacshunds, etc.” — flipester
They wouldn’t even notice if they destroyed us completely
“One theory is that if they are sufficiently advanced that aliens may treat us like ants. When we build dams, we don’t worry about whether or not a dam will cause an ant hill to be flooded out. Similarly, a species that is advanced as far beyond us as we are beyond ants might seek to alter our planet or even our solar system to their advantage without giving consideration to what may happen to us.” — Victor_HardApple
It would be safer for them to get rid of us
“This reminds me of the Dark Forest Theory. Like hunters in a “dark forest”, a civilization can never be certain of an alien civilization’s true intentions. In summary:
~All life desires to stay alive.
~There is no way to know if other lifeforms can or will destroy you if given a chance.
~Lacking assurances, the safest option for any species is to annihilate other life forms before they have a chance to do the same.” — 2020Chapter
It could just be another pandemic
“For all we know, aliens might not be the ‘little green men’ that fly around in flying saucers and destroy us with laser beams – they could be an interstellar pathogen that show up one day and silently and effortlessly kills us all without warning. Our immune systems would have no idea what hit them.” — CaptainWisconsin
Completely different morality
“Our human concepts of morality and empathy are heavily influenced by our mammalian biology.
Imagine that aliens land, and instead of having nice little family units their species lays clutches of thousands of eggs at a time. They don’t form strong bonds. Life is essentially expendable for them. They see us weeping over a dead child and they have literally no frame of reference for understanding why this would be upsetting.
I’m not saying all aliens will be like this; but some definitely could be.” — ToBePacific
Our worst fear
“What if there really is valuable information to be gained from anal probes?” — HarlanCedeno
The monsters could be us
“What if it ends up the other way around? What if we end up being able to travel between the stars, and we find some aliens that aren’t as technologically advanced as we are? I could easily see us being imperialist invaders, committing the atrocities we’ve committed against each other on an entire species so we could have a second planet with an oxygen atmosphere and easily attainable natural resources. That would be truly horrifying.” — Victor_HardApple
They may not care about us at all
“The scariest thing to me is thinking that aliens want nothing to do with us.
I don’t get why everyone thinks that aliens would attack us immediately. It’s one thing if we come into their territory, but if they stumble upon us we’re most likely safe. Them finding us would mean they are incredibly advanced. Anything they could get by attacking earth they could get far easier by harvesting it from a different planet in our solar system. Unless they feed specifically on brainwaves, it isn’t worth the hassle to attack a planet teeming with life they may have some weapons, however primitive, that might be used against you.
It’s far more frightening to believe that aliens have/will one day discover us and won’t be interested, They won’t want us on their intergalactic counsels, they won’t want to give us medicine, they won’t want to form any bond. If they decide to just leave humanity alone, that means either they have decided we are not worth saving, or that we are too close to destruction to be worth the resources.” — BombsNBeer
They’re vaccinating themselves against us
“Maybe all these supposed alien abductions and UFOs over the past 100 years have been alien scientists building vaccinations against our known diseases for themselves and adapting their own medical immunities for humans too – monitoring our progress against our own diseases so when both civilizations are on top of it we can be welcomed into the galactic fold safely.” — [deleted]
We won’t even see them coming
“That one day they will just wipe out our planet without us ever knowing why and that it will be so fast we don’t even realize it.” — Jenova66
What it means for us if aliens aren’t real
“That they don’t actually exist. If there were no advanced civilization that can colonize other planets, that would mean that most of them go extinct before they reach that level of technological advancement, which means that we are probably doomed to extinction too.” — bullet-cat
We’d be livestock to them
“Maybe they treat us like we treat pigs, cows and chickens.” — HansMustermann
They may not poop
“They might not have orifices for eating or pooping.
Hear me out. Almost all life on earth is topologically equivalent to a donut. Meaning we have an outside wall and an inside wall (digestive and respiratory track) that runs like a hole through us, and our important bits are sandwiched between these walls.
One of the only examples of life that isn’t like that would be for example single-celled organisms, amoebas and such, which just kind of ooze over whatever they acquire sustenance from and osmos it into themselves. Currently on earth only micro organisms are like that (edit: also some weird ocean creatures).
But there are fossils dating from before the Cambrian explosion from millions of years ago which hint that a completely different type of large and complex organisms might have evolved to walk the earth before that evolutionary lineage was wiped out by an asteroid or the like. One of the oldest of these rare fossils is of a zero-orificed creature that was about a meter long! We don’t have a lot of info from life during that age but it indicates there may have been an entire evolutionary track of complex life forms favoring life that is topologically equivalent to ball instead of donut. And that’s on EARTH.
So, yeah. We could very well find an entire planet of giant, oozy amoebas of assorted species and intelligence, who neither eat, speak, or shit like we do. Just try figuring out how to beach THAT cultural and language barrier.” — Maxwells_Demona
They’ve already noped out
“That they’ve found us, observed from a distance for the past ten years or so, and have declared the planet a disaster area that nobody should ever visit.” — CaptainsLincolnLog
“You know those tribes that live on islands in the Pacific that haven’t had technological progress in thousands of years? That’s us.
You know those international logging companies that would love a chance to just come plow over the island and take what’s there?
Somewhere, there’s an alien Greenpeace that’s just barely getting by on a shoestring budget.” — buyongmafanle
The Dark Forest Theory
“The Dark Forest theory kinda freaks me out. The universe is a dark forest. Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound. Even breathing is done with care. The hunter has to be careful, because everywhere in the forest are stealthy hunters like him. If he finds another life—another hunter, angel, or a demon, a delicate infant to tottering old man, a fairy or demigod—there’s only one thing he can do: open fire and eliminate them.” — Juturna_