When you do an experiment, you don’t really know what you’re going to find. So if your equipment works and isn’t fidgety and gives consistent results, the results, whatever they may be, constitute a success of sorts. You’ve got an answer to your question. Maybe not the answer you expected, but an answer.
Today’s efforts to find dark matter are another example. We don’t know what it is, but at least we know a little bit more about what it isn’t.