Is there a substitute for dark rum?
In that we have already provided the definitive answer to this question elsewhere (relying on rum extract), we assume your follow-up is seeking a non-alcoholic substitute for rum (as extracts can be as alcoholic as or more alcoholic than the spirits they replace).
We have no idea what you're making or how important the rum flavor is to the finished dish, so finding an acceptable substitute is going to land mostly on your shoulders. You can try water, white grape juice, pineapple juice, apple juice, or apple cider. Your recipe's call for dark rum, though, is surely an indication that it wants a lot of flavor, so all of those may fall disappointingly short.
Rum is distilled from molasses or cane syrup, so your best bet could be a mixture of molasses thinned with pineapple juice or white grape juice.
Substituting Rum Extract for Rum