Depending on what you're contrasting cargo with:
* Passengers: If you're talking about the things being transported, the antonym could be "passengers" if the mode of transport is carrying people instead of goods.
* Empty space: If you're emphasizing the absence of cargo, you could use "empty space", "void", or "nothing".
* Lightweight: If you're contrasting the weight or bulk of cargo, you could use "lightweight" or "light".
* Non-essential: If you're focusing on the importance of cargo, you could use "non-essential" or "unnecessary".
Considering the purpose:
* Destination: If you're considering the purpose of cargo, "destination" might be a suitable antonym, highlighting the act of delivering something rather than the goods themselves.
Ultimately, the best antonym for "cargo" will depend on the specific situation and the meaning you want to convey.