If it is matter at all, then we do know something about it, that it is "dark" (does not interact with electromagnetism). The alternative is that it isn't matte

If it is matter at all, then we do know something about it, that it is "dark" (does not interact with electromagnetism). The alternative is that it isn't matter at all, and that some other explanation is required for galactic rotation curves, the Bullet Galaxy, observed gravitational lensing, and the configuration of the cosmic microwave background. It's nice that one theory currently does all of that. That doesn't guarantee that it's right, but it means it's more than just "we have a guess about one thing".