Dark matter is a hypothetical form of matter that is responsible for the observed excess of gravitational forces at galaxy scales. It's mass is currently estimated to be around 5x the mass of ordinary matter. This question is resolved as soon as one or more constituents of dark matter are discovered such that the remaining unknown part of dark matter is no more than 2x the mass of ordinary matter.

If dark matter is composed of some particles like axions or WIMPs, standard criteria for particle discovery are applied.

If dark matter consists of macroscopic objects like primordial black holes, at least one of them has to be discovered, and it has to be shown that objects like that can account for a big enough share of dark matter.

If dark matter is explained by some modifications to the laws of gravity, the new theory has to be widely accepted, the Wikipedia page for General relativity has to point to the new theory as the one that supersedes GR as the accepted description of gravity.

I do not bet on my own questions.