I don't want to live on this planet anymore.
Why it is hard sci-fi
A comedy. The shows writers room contains a math PhD and the show commits to its own physical jokes with rigour. The What-If Machine has internally consistent rules. The hyperdrive is a working engineering problem. The shows famous body-switching episode includes a published mathematical theorem about how to undo the swap with a finite group of additional swappers, which is the only theorem written for a sitcom.
Science inside it
Original mathematical results, a working faster-than-light grammar in which the universe moves rather than the ship, a careful approach to time travel as forward-only with rare exceptions, and a recurring engagement with information theory in episodes about the I Hop and the smelloscope. The show is rigorous when it wants to be.