https://en.wikipedia.org/wiki/Baryonic_dark_matter

This resolves once we have a definitive explanation for dark matter that is not contested by any significant fraction of the physics community.

In the event that the phrase "dark matter" comes to mean something else (such as our observational methods improving to the point where we can directly detect some forms of dark matter and we no longer call them "dark"), the 2022 definition is what will matter for the purposes of resolving this market.

In the event that dark matter doesn't exist (e.g. modified Newtonian dynamics), this resolves N/A. I'll be pretty liberal in terms of what counts as "matter" though, in the event of disagreement over whether some new particle should qualify.