I recognize the sort of phenomenon you're pushing back against here, and there's legitimate value in consciously trying to counterbalance the all too human tendency to nod along to overconfident proclamations of truth.

But even so, the claim you're making here is absurd when treated as a general rule. Expertise is a real thing that people can have. Yes, there are countless places where a genuine expert will express caution and give carefully qualified statements about what is "true", and that's good. It's especially common for topics close to the frontiers of human knowledge. But there are also a whole lot of places where an expert will be willing to say "it's basically safe to take ____ as true", and where if asked they'll be able to back that up with copious explanations and evidence.

If I tell you that tomorrow morning the sun will rise in the east rather than the west, I will say it very confidently, and I will be right. If I tell you as a physicist that this bowling ball I'm about to drop from the second floor window will accelerate toward the ground at a rate very close to 9.8 m/s^2 (maybe only a smidge less due to air resistance, given the short time involved), I will be right. You trust your life every day to countless engineers who have at some point said, "The design and fabrication process we have implemented has involved enough rigor and enough quality control and enough margin of safety that I'm willing to stake my career on it being safe." And the vast majority of the time, for each essential bolt in your car and each fuel line on your airplane and each support beam in your office building, they are right.

So here, in the case of dark matter, the community of astrophysicists and cosmologists has assembled over the past 50-ish years a tremendous array of evidence from multiple independent directions that all seems to paint a consistent picture of some type of matter with quite specific properties. Many of them were highly skeptical at first, and there are still some actively looking for radically different explanations. But the vast majority of those experts have considered all of this positive evidence for "dark matter" and have become convinced that something very close to the Lambda-CDM model must be more or less true.

It's not at all clear to me what more you want from them. I mean, heck, the gravitational lensing results (especially as applied to something like the Bullet Cluster) could be considered a successfully tested prediction of the dark matter model, since those measurements weren't even possible until well after most of the community had become reasonably convinced that dark matter was real. The fact that we haven't yet answered the question "how does dark matter fit with the rest of our understanding of particle physics and astronomy?" is fascinating, but the existence of open questions doesn't on its own mean that all that observational data somehow doesn't count!