Ars Technica has a very good article on the role of placeholders in science:

The comments appear like clockwork every time there’s a discussion of the Universe’s dark side, for both dark matter and dark energy. At least some readers seem positively incensed by the idea that scientists can happily accept the existence of a particle (or particles) that have never been observed and a mysterious repulsive force. “They’re just there to make the equations work!” goes a typical complaint.

It’s a somewhat odd complaint. Physics has a long history of particles that were predicted based on the math and not detected for years, sometimes decades. But it’s not simply physics. Other areas of science have produced evidence that suggests something must be present, but haven’t hinted as to what that something must be. These situations, where scientists insert a placeholder for a something they don’t understand yet, have sometimes led scientists down the wrong path—phlogiston and aether spring to mind.

But these erroneous placeholders carry the seeds of their own destruction, since they make predictions that the natural world can’t fulfill. And, possibly more often, the placeholders turn out to be right, and an understanding of the phenomena behind them revolutionizes our knowledge of the natural world. In this feature, we’ll take a look at some of the most successful placeholders in the history of science, and then consider how even a placeholder that has gone wrong can help advance a field anyway.

Worth a read to understand that the use of placeholders is how we make progress in science.