"Many scientists believe that if Einstein hadn't incorporated gravity into the theory there's a good chance that it would still be undiscovered today." Actuall

"Many scientists believe that if Einstein hadn't incorporated gravity into the theory there's a good chance that it would still be undiscovered today." Actually, no. If you start out with flat spacetime & try to describe massless spin-2 particles, you'll find gravity falls right out of the equations. It's quite elegant, really.