Question

Lacking direct evidence of WIMPs as dark matter, why must we eliminate all other possible explanations based on the known forms of matter before we invoke their existence?

Short answer

Because we must not complicate our theory by introducing additional particles if a better explanation can be found using existing forms of matter.

Step-by-step solution

Definition of WIMPs.

A WIMP has no precise description, but it is a new elementary particle that interacts with gravity and any other force (or forces) that is as weak as or weaker than the weak nuclear force, but non-vanishing in strength.

Explaining Occam's razor.

Occam's razor is a problem-solving principle that states that if two theories provide equally precise explanations and predictions, the one with fewer assumptions is the "right" one.

With this in mind, if dark matter can be explained using existing particles, Occam's razor suggests that there is no need to include new particles, as this would complicate our hypothesis unnecessarily.