I remember that the last scene of the show has the group of astronauts talking or arguing about something and they are surprised by the entrance of someone who was conducting the experiment on them shows up because they had thought they were in a spaceship or space station the whole time. Not sure if anyone in the group was in on the experiment.
It could be Ascension. This was a 2014/2015 miniseries. It was aired as either 6, 1 hour long episodes or 3 feature length instalments.
The story takes place aboard a generation ship originally launched in the 1960s and now half-way into its 100-year journey to Proxima Centauri. A murder on board sparks off a series of events that leads the crew to begin second-guessing their real mission.
The Ascension is the titular spaceship. It has a large crew and strict birth control laws and dual military/civilian control hierarchy. A significant amount of tension exists between the Upper and Lower "Decks". One living a 1960s university campus life style, the other grease covered engineers and manual workers.
At the end of the 1st double episode it's revealed that the spaceship never left Earth, it's parked in a basement somewhere, but it takes the whole rest of the series to unpack the real purpose behind the arrangement.
There are a couple of instances where people see things they're not expecting to. But for the most part everything is accepted at face value, the opening plot threads focusing on a murder committed on Ascension, with a gun that shouldn't be there.
One person on board the spaceship was in on it. Everyone else genuinely believed they were on a generational interstellar spaceship.
It could be the British series/reality show Space Cadets from 2005.
Space Cadets is a British television programme made by Zeppotron (a division of Endemol UK) for Channel 4. Presented by Johnny Vaughan, it was aired across ten consecutive nights beginning on 7 December 2005, with the final episode aired on the evening of 16 December 2005.
The series was a hoax at the expense of its contestants, who were told they were being trained as cosmonauts at a Russian military base before undergoing a five-day trip into low earth orbit. In reality, the entire series was filmed in Suffolk, and the contestants did not leave Earth. The series was accompanied by a behind-the-scenes sister show Space Cadets: The Satellite Show, with interviews and phone-ins.