Can Dark Matter be the energy source of the future - a rough estimate

[Alex K]

Since dark matter might only interact with the weakest of the four forces, gravity, that would likely mean converting it into useful energy would be less efficient than converting other forms of matter which interact more strongly with the other three forces.

Yes,this weakness of interaction is a (I'd say the) major hindrance and a big if . But there is a good chance that one can convert the mass of the DM to pure energy with 100% efficiency. That's not possible with ordinary matter.