Professor Brian Cox has revealed the one thing about our universe that 'keeps him up at night'.
Space is a mysterious thing - the vast unknown that is constantly expanding away from us holds a lot of unanswered questions.
Scientists have been studying the stars and the night sky for centuries and centuries, but there's still so much we don't know.
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Are we alone? Is the Earth flat? OK, maybe we know the answer to the latter, but humanity hasn't even stepped foot on another planet yet.
Cox has even given his take on the whole (absurd) Flat Earth theory.
Spoiler - he thinks it's a load of rubbish.
The British physicist has previously also hinted at the fact that aliens may not be real after all, and explained what could happen to a human if they were to be sucked into a black hole.
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But in a YouTube video for BBC Earth Science titled 'Brian Cox On The Most Terrifying Places In Our Solar System', he revealed the one thing he loses sleep over when it comes to our ever-expanding universe.
When asked what he thought of the 'possibility of discovering alien life', Cox answered: "I would be delighted if we found life elsewhere because I feel it would take some of the pressure off."
The 56-year-old added: "My fear, the thing that keeps me awake at night, is that life is extremely rare in the universe and complex life is so rare that we might be the only planet in the Milky Way galaxy where complex life exists currently."
Cox went on, explaining that it's a 'guess' and a 'possibility', not claiming it to be fact.
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"I'm not saying it definitively because we don't know so if that's the case," he highlighted.
The physicist added that if there was someone or something else out there, and if a flying saucer landed on the interview they were carrying out in London, he'd be 'relieved'.
"[I'd be] like, 'thank God that it's not only down to us now', because you know, we're not making a very good job of it," he claimed.
While scientists are still exploring space, there is still no concrete evidence pointing towards extraterrestrial life within our solar system, or beyond it.
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Despite admitting that he thinks within our galaxy, Cox has theorised about the idea of aliens on social media.
After speaking about Voyager 1 and how spectacular it is that it can be fixed from Earth, he received sarcastic comments from some users, which he labelled as 'stupidity' and an example of our 'increasingly dim-witted civilisation'.
Cox then spoke about the Fermi Paradox, which is the contradiction between the lack of evidence for extra-terrestrial beings in the universe against the high estimates of their probability.
"Until recently my guess has been that the answer to the Fermi Paradox might be found in biology - complex biological systems are rare. I’m increasingly of the view that the reason for The Great Silence is that civilisations are inevitably crushed by the weight of n**heads shortly after inventing the internet," he concluded in his well-known sarcastic tone.
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Never change, Brian.
Joshua Nair