They don't preach these things. Physicists don't run around preaching unprovable holy grail theories. They just make something up. Out of nothing. Because its their job. They need to come up with falsifiable theories that can be verified in lab experiments.
In order to do so, they HAVE TO hypothesize something. They can't just say "err dark matter right?". They have to say "there could be a particle with these properties, a mass of roughly X, producing a cross-section of Y when bombarded with tau-leptons etc.". That's just the scientific method.
Any physicist who postulates particles does that from a position of "it probably doesn't exist". The same way when you measure something to be faster than the speed of light, you assume that a cable is broken.
Then some journalist comes around saying "lol paper says weird particle exists, scientists are whack". That's annoying. But it happens.
If journalists wrote about CS the way they write about physics, the next time someone loses 2 BTC from their wallet, they'd claim that thats hard evidence of quantum computers being invented by north korea. Hypothetically plausible but as outlandish as permissible by reality.
https://en.wikipedia.org/wiki/Dark_matter#Composition_of_dar...
It might be ordinary matter hidden from us somehow (maybe inside black holes) or something new like sterile neutrinos or WIMPs.