The laws of the universe.
Knowhere at any time, has anything, ever, broken the laws of the universe. Galaxies, stars, blackholes and even the big bang, the most gigantic energy events in the universe have still never broken the laws of the universe. There's an upper limit that cant be breached and we're still messing about at less than 0.01% of C
The faster you travel the more mass you gain, the more energy it takes to move that mass, that bit faster means more mass, means more energy, means more .... till eventually to move that mass just 1mph faster would take more energy than is contained in the universe itself.
It's the old
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Warp engines and hyperspace are just cool sci-fi deus ex machina plot devices