Humans are a protected species, carefully raised and nurtured by higher organisms that are hundred thousand times larger than us (in terms of space and time). The earth and solar system is just a vivarium of galactic scale. Several "glass wall" mechanisms were placed to ensure we are separated from the rest of the universe, like the oort cloud.
Somewhere out there in the universe are humans living freely outside the glass wall.
Playing with this outlandish thought further, the idea that earth is an enclosure for research or preservation explains why the solar system is configured the way it is and why tiny adjustments could render it an inhabitable wasteland.
As time goes on, the glass walls tend to age and occasionally the "cracks" allows external lifeforms (aliens) to slip past the protection layer, much like how insects or parasites would occasionally slip past any opening.
With this in mind, there would be two types of aliens/visitors to our enclosure: (1) lifeforms that are similar to humans in terms of size and biological complexity but more technologically advanced and (2) our protectors who are incomprehensibly larger and more complex than any life on earth. This isn't as outlandish as it seems to be considering there are stars that are million times larger than our own sun.
This sheer difference in scale might as well make the protectors exist in a different dimension. This explains why people would say that aliens were here all this time, but from another dimension. Technically they are here, but more accurately to say the protectors surround us.
Occasionally protectors would step in to prevent mankind from destroying their own enclosure, much like a terrarium owner would reach into the enclosure to remove unwanted organisms. Frequent unidentified sightings on nuclear sites is the protectors' way of peeking and reaching a "hand".
It remains to be seen if humanity would one day evolve to outgrow the enclosure.
Humans are a protected species, carefully raised and nurtured by higher organisms that are hundred thousand times larger than us (in terms of space and time). The earth and solar system is just a vivarium of galactic scale. Several "glass wall" mechanisms were placed to ensure we are separated from the rest of the universe, like the oort cloud.
Somewhere out there in the universe are humans living freely outside the glass wall.