a suitable form of exotic matter (negative energy) can be harnessed
It is highly unlikely that humanity will ever have exotic matter, but there is a small sliver of hope
The Casimir effect shows that quantum field theory allows the energy density in certain regions of space to be negative relative to the ordinary vacuum energy, and it has been shown theoretically that quantum field theory allows states where the energy can be arbitrarily negative at a given point. Many physicists such as Stephen Hawking, Kip Thorne, and others therefore argue that such effects might make it possible to stabilize a traversable wormhole. Miguel Alcubierre has suggested using the effect to obtain the negative energy density required for his Alcubierre Drive.