Earlier quoted context omitted.
What do you mean there are “no constraints” on dark matter. There are plenty of constraints. I think you just have no idea what you are talking about.
Calculations involving dark matter might as well be an experiment in curve fitting. We won’t find dark matter in your or my lifetime, because it doesn’t exist. It is a convenient, flexible, and inelegant fudge factor.
No, not really. We can observe it indirectly and have mapped out its structures. And intuitively, why do you find it implausible there is a massive particle that only interacts via gravitational force?