Or you manage to store it somewhere. Thinking extreme here, we can get massive energy out of nuclear fission, what if we could revert it and put massive energy

Or you manage to store it somewhere. Thinking extreme here, we can get massive energy out of nuclear fission, what if we could revert it and put massive energy back into elements to do reverse nuclear fission. Or maybe convert lead into gold. And use it to create fuel to power our space trips.