Neil deGrasse Tyson is, in case you somehow didn't know, is a genius. Aside from that, he has also dropped knowledge bombs on Colbert and his audience nine times now! This time, he revealed what keeps him up at night.
With someone as smart as Neil, we all have got to assume that what keeps him up at night is something to do with his mind full of astrophysics. Lucky for us, we're right!
The interesting part though, is the fact he loses sleep over the fact "we don't know what we don't know."
From our understanding, Neil says that the universe is expanding so much that soon it will expand to the point where the galaxies and stars we know now will be pushed out of our view in the future. He says future "cosmic explorers" will have no chance of knowing what we have known and studied now.
This all also gets him thinking about how this has probably happened before and there's a whole area of space we know nothing about so there's genuinely a possibility we have no idea about things we don't know about.
He then compares it to touching an elephant that maybe isn't really there and something about it's shadow. We're not sure. After all that, we might be losing sleep too from pure confusion!