Are you familiar with dark matter? It's one of the biggest mysteries in science; if not, wiki is a good place to start, plus it's about all most of us can understand anyway.
(Notice that above I said "familiar" with it, not "do you understand it", because I certainly don't. I do understand that it can't just be regular non-luminous matter; e.g., gas clouds that we can't see because they don't give off light, or if you're as nerdy as me, high-Kardashev-level aliens that have made a bunch of Dyson spheres.)
So why can't dark matter just be hard-to-detect regular matter? Because various things we're pretty sure about in the universe match certain things we observe (like, the distribution of elements matches what we think would have been made at the Big Bang) but they don't match other things (like, the microwave background and the amount of gravity holding things together). And it's not just a question of rounding up to get the right answer - based on our current understanding, most of the matter in the universe must be dark matter!
My reason for posting this is that the science fiction writer David Brin points out that there's a competing theory from one group of physicists, which is that essentially macroscale (Newtonian) physics behaves differently at low forces, and voila, dark matter problem solved. This is not anywhere nearly widely accepted, but Brin's point is that while there are many crackpots railing at the establishment for suppressing their brilliance - physics crackpots, and biology crackpots (we call them "creationists") - but here we see there's a difference between crackpots and a genuine non-mainstream theory. The physics establishment isn't necessarily signing on to the new explanation for dark matter, but it's a serious enough attempt that it's being evaluated seriously. Creationism, not so much. (And again, creationists, why not bypass the journals? Why not just found your creationist biotech company and cure cancer? Somehow that never seems to happen!)
So, next time someone says "There's a conspiracy to surpress the truth, and that's why I can't get my theory published" - you can point them to this example, and tell them sorry, it's just because they're a crackpot.
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