According to astronomer Greg Aldering, the scale of the void is such that "If the Milky Way had been in the center of the Boötes void, we wouldn't have known there were other galaxies until the 1960s."

If your aliens live on isolated star(though it would be hard to explain - see star formation process) in the middle of void then they simply would not see anything until they get telescopes powerful enough.

Maybe the aliens' planet is perpetually overcast, like Venus or Titan. They may be able to deduce that their planet has a sun from the day-night cycle and how the brightest spot on the sky moves, and they may also deduce the existence of a moon, if it is bright enough, but they would not be aware of the existance of other stars or planets. This would of course require that no planes had penetrated the cloud cover, but if it is high enough, that might not be a problem.



It may also be that the planet is tide-locked, always facing the same side towards the sun, and only the sunny side is habitable, the rest being far too cold, even for planes. With the sun perpetually high in the sky, stars would be nigh-invisible.



The planet may also be orbiting a multiple star system, and there is always at least one sun in the sky, clouding out stars. Isaac Asimov used this in his classic short story "Nightfall".



Finally, the solar system may be situated in an interstellar cloud of dust or gas that obscures other stars. The light from the sun might even make the dust or gas glow, further obscuring any stars.



Otherwise, if the stars are visible, even technology far below 1950s level would show that stars are distant, very bright objects similar in nature to the planet's sun.



Of course, it may be that your aliens don't rely on sight, but rather sonar or some such. They might still have a limited sense of sight that can detect major light sources, but isn't precise enough at a distance to make out stars or other planets. Basically, they are all very near-sighted.

There is a way to do it, but it comes with a hefty price. Put your solar system in a nebula or other dust cloud that (mostly) obscures the stars around you.

The reality is that things like stellar parallax are always going to get the better of your scientists because there's simply a more compelling argument for the universe extending beyond the solar system. Even with rudimentary experimental observations, it's hard not to draw the conclusion that the universe is vastly larger than the solar system.

Hiding all but your brightest night time stars though helps mask this problem, especially if the dust in the clouds drifts a little, sometimes blocking out one star, other times blocking out another. Many of the observations that were made of the stars in early times were only possible because we could always see them, meaning that we could tell which stars were moving, and at what speed, and how their course 'drifted' across the sky through a given year, meaning that we could learn a lot about our universe even without telescopes. But, if you have a strong dust cloud say at the same location as our Oort cloud or even Kuiper belt, then it might be harder to even know that the star we can see today is the same star as the one we could see over in this other place in the sky 3 months ago, before the cloud shifted a little.

Arguably, many may well end up believing that the dust cloud contains certain highly reflective elements or rocks, that cause it to sparkle in different locations, when they turn just so and reflect the sun back onto the habitable planet.

The trick here is not limiting theory; science does that far too well. It's limiting observation. Make it hard for the observations we've been making since very early history to be made on your planet, and the theories won't form because there's no observations on which to base them. Of course, the only way to limit observations of the stars is to obscure them.

Whether or not that can be done with a nebula in prime, or needs a specific form of dust cloud or other barrier is a deeper question, but from a position of strategy, the easiest way to make your people believe there is a limit to the universe is to hide as much as possible from beyond that limit from their abilities to observe.

I'm surprised nobody answered that on WB.SE, but:

Wouldn't a big Dyson's Sphere do the job?

I remember reading about a Dyson's Sphere large enough to collect all the energy from a solar system, on another thread. This would effectively block all outside interferences.

But this solution depends on what you want in your story. (If you don't want to go hard sci-fi, it will be hard to feature it).

(If anyone is knowledgeable on the topic, feel free to correct me, I only know the general principle)

Bad eye sight

Humans have really good eye sight, and this is why we know about the stars.

There are three aspects to this, first there is resolution, meaning we can see small dots in the sky.

Second there is near-/far-sightedness. Not every organism can focus beyond a few meters. You don't have to be very far out of focus before the stars simply disappear.

Third, we can see well at night. This is not automatic. Stars doesn't really give off all that much light. If your xenos simply hide at night rather than trying to function, they are likely to be practically blind.

In addition you probably shouldn't have a really big star like Sirius be as close by as Sirius is to us.

Note that these xenos are also likely to miss planets for the same reason.

A species that has poor eye sight probably has other senses that are better: hearing, smell etc.

I will push @StigHemmer's idea a little bit further:

They are completely blind.

They might possess some form of synesthesia, allowing them to "see" smells, and even combine that with some infrared sensibility. Heck, even some echolocation like dolphins or bats. With that, they might "see" the sun, even the moon if it is extremely reflective. They can hunt, they can have relationships, they can understand nature. They might build a civilization just fine, even developing some sort of 'writing' based on smells. IMHO it's perfectly possible. The only thing I don't know how they will achieve is flight, but I imagine they would HEAVILY rely on radar.

One situation that could cause this for our alien friends (and for us as well) is the fact that the expansion of the universe is speeding up (accelerating). At some point in the far future, the universe will be expanding faster than the speed of light. At that point, no light from any of the other galaxies in the universe will ever reach the aliens. Every experiment they perform will conclude that there is nothing else out there except for their local group, which could still be held together by gravity.

A rogue solar system in interstellar space might fit the description (kind of like a rogue planet). For example gravitational interactions in the past launched the solar system outside of its parent galaxy, and now it is just floating in space faraway from any light, or galaxy. Your species reached scientific maturity after the solar system already disappeared into interstellar space.

In this case the solar system might seem like all that exists, since everything else would not be visible. However in this case the solar system is functionally isolated, and is basically the only "universe" they need to care about.

Could an intelligent species form this hypothesis, or would other scientific principles mean that no thinking race would believe it by this technology level?

If the alien species follow the scientific reasoning (aka make a model, use the model to make a forecast, check with experiment if the observation validate the forecast: if they do, make another forecast, if they don't, discard the model), their model won't stand the observation.

That's exactly how we got rid of the ether hypothesis: experimental evidences were against the forecast made by the ether model, so it finished down the drain (Michelson and Morley experiment)

Stars and other visual and electromagnetic effects from outside their solar system are explained as complex reflections and optical illusions.

I assume, without further details on this model, that sooner or later it would become observable that:

• the level of detail of these "artifacts" is extremely fine and not matching any known, in system, source.


• the appearance of these "artifacts" is not affected by the relative position of the observer


• the "artifacts" have a red shift, so whatever is causing them is moving outward, hinting that there is space out there.

The Bootes void is a good plan. There are issues; having a star isolated near the middle is tricky, as stars (especially metal-rich ones) require other stars to form, and don't move that quickly. So the other stars formed with that one would be much closer.

If we ignore that problem, Naked eye astronomy is apparent magnitude 6.5 or so. Naked-eye galaxies are that bright at about 15 Mly away. Telescopes are going to extend that much further.

In 1900 telescopes maxed out at 1.25 m, or 200x larger than the human pupil. The limiting magnitude is then about 6.5+11.5 or 18.

We can presume less interest in telescopes (especially huge ones), so using 1900 numbers for a 1950 society is not unreasonable.

And things barely visible with a telescope might be dismissed as artifacts of some kind; galaxies where spotted for decades before they realized they wheren't local to our galaxy. A similar issue might happen here.

Anyhow, that works out to things roughly 200 times further away, or 5 Gly, or 20 times the size of the Bootes void.

You'd have to limit telescopes to something like 5 times a human eye -- and a telescope that small is going to be used for navigation, let alone astronomy -- to avoid being able to see the edge of the void from its center.

Still, it might take time to work out that those swirls are in fact not visual artifacts.

In theory placing yourself in a dust cloud also helps; but dust clouds generate stars (which also explains your star).

Now, if you want to talk about a local cluster of stars, that become more practical. Aforsaid dust would reduce the apparent magnitude of the galaxies far away, and there would be a handful of close stars. Those stars would all seem to be orbiting each other, and beyond them would be a void.

Unfortunetally being able to see they lack planets becomes a challenge.

What more, in 1950s, people where uncertain if other planets in our solar system lacked (complex macroscopic) life; if you make telescopes worse in your world, they become even less certain; and if you make them as good as our 1950s telescopes, hiding the galaxies outside the void becomes less plausible.

Make their vision centered on the ultraviolet spectrum

This could easily work out if the atmosphere of the planet is humid, with a lot of water vapour, which is opaque on the UV (hence the reason we can't do UV astronomy from ground). Then you would have no problem in developing all kinds of technology in a very similar fashion to how they evolved on Earth. With a vision centered on the UV, instead on our "visible" band, they would see their atmosphere as a cover, and probably wouldn't even imagine that there are other suns.

But, then, when they began launching satellites (maybe for military purposes) and began flying higher (the SR-71 Blackbird flew in the 60's) where the atmosphere is much thinner, they can start to wonder why are those bright specks on the sky, hence discovering that there is a huge cosmos out there.

With those reports, they start to design a military survey satellite to "look the other way", i.e., outside, and not for the ground. And, boom, they discover astronomy ;-)

The are alone in their observable universe

At some point in the far future, other galaxies will move outside of our observable universe.
If this star system was actually a rogue star, not belonging to any galaxy, they may be completely alone in their observable universe.

Of course this isn't very useful if you were planning to give them a reason to change their mind about it later.

I think their are several good answers here already, but it might be useful to categorise them a little by theme.

Theme 1: The aliens cannot see
- As suggested by: Alexandre Aubrey, Eduardo W., tfrascaroli

This includes the idea of a species who rely on echolocation (or similar) for everyday "vision" and thus do not detect light naturally. By the 1950's one might expect them to have invented machines to detect light for them - but exactly when a light detector would be invented by a hypothetical blind species is obviously very speculative.

This blindness fits neatly with other environmental reasons. If they live on a dark planet under a thick aborning atmosphere it makes sense that eyes would not have evolved on their planet. Similarly, they might live on a planet very close to its host star, where the surface is so scotching hot that all life emerged and lives deep underground (hence blindness). By 1950's level tech they may well have only just finished a "surface race", IE. which nation can be the first to build some kind of suit that can survive the blazing surface and put a person on it. (The poor surface-nauts later died from radiation poisoning).

(Explaining the thermodynamics of a planet that is hotter on the outside than the inside might be problematic, perhaps it is in some unstable orbit where it spends a few thousand years getting baked then a few tens of thousands away from the star and cold, and they so happen to have "teched up" in the hot stage.)

The star system is in an atypical location
- As suggested by: Joshua Kearns, Yakk, Vashu

The star system may be extremely isolated for some reason, or more likely might be inside a cloud of interstellar dust that obscures nearby stars.

Philosophical or religious barriers
- As suggested by: Thorne

These aliens may have strong cultural or religious norms concerning the stars. Their are many ways to spin this, anything from a (probably unrealistic) worldwide cult that insists that the stars are ice demons (as in the old Doctor who episode) and that looking at them or thinking about them might tempt them to eat your soul.

You could of course spin it almost backwards, they are very utilitarian. If you are doing something that doesn't seem useful you are ostracised and cast out. What could be more useless than absentmindedly looking at the sky?

In this category you can also place the "mind-boggle" of scale. As the Hitchhikers guide puts it "Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is." Perhaps this species very much don't believe it, in spite of the evidence they may have difficulty wrapping their minds around it - preferring instead to accept that their solar system is alone and cooking up dodgy theories to keep themselves happy.

This will undoubtedly happen at some point in the future. The universe is expanding faster than the speed of light. As time progresses, the amount of systems in our observable universe will decrease. If you were born far enough in the future, you might think your galaxy was all that existed. Further still, your solar system could be the only thing in your observable universe. It is pure fluke that we have become sentient early enough to see how non-unique our situation is. For a civilisation born a trillion years in the future, there could just be its solar system, and a much cooler Cosmic Microwave Background. It wouldn't even be a blunder for them to hypothesize that they were all there was, and they'd need far more advanced technology than ours to realise they weren't.

This idea would probably change a lot of the setting, but...

An aquatic species that lives deep enough to have little to no sun would be unable to see the stars. Perhaps their planet is even covered in a sheet of ice.

Establish your people near volcanic activity and you can use geothermal effects to generate electricity.

Electronics could be developed by finding a way to create vacuums to hold the electronics away from the water. Eventually, they can develop waterproof coatings and manufacture in vacuum, cover in waterproof coating and then it can be used in the water without the vacuum.

Or perhaps a land-dwelling species that retreated under the seas by building a large air-filled dome on the sea floor (perhaps to escape radiation from a nuclear fallout) and, over time, lost knowledge of the world above. This would require them to have had sufficient technology to create this dome prior to retreating to it, and then live in it without much advancements for long enough that people forget about land.

They could happily live without advancements of tech for a long period of time because they just saw that such advancements led to the nuclear weaponry which forced them underwater. R&D would be taboo. They would stay underwater because legends of the deadly air keeps them from going back to the surface, or conspiracy theories about the non-existence of the land become the norm.

The land-dwelling species would also use geothermal effects to generate electricity, and use that electricity to generate oxygen by hydrolysis (reducing the need for plants to generate oxygen). Their electronics would be manufactured and used within the air dome, no need for extra protection from water. They could use scuba gear to scavenge food (fish, seaweed) and materials (metals, salt etc.) to survive.

Iain Banks' Against a Dark Background is set in a solar system with several settled planets and 10,000 years of history. Only late in the novel is it revealed the entire system is in interglalactic space, and no stars are visible in the sky. It's a lonely feeling, knowing all these interesting characters will never leave their home system, because there is nowhere to go.

And The Culture will never find them, either.

Religion

The local indigenous religion states that God made created the solar system and the stars are just lights to brighten up the night sky and light the way for travellers.

If religion can make humans think the Earth is flat, this religion is already closer to the truth.

The general population really doesn't like their religion to be challenged so nobody would even be looking.