There are many questions which come up again and again, virtually every day, trying to propose an alternative explanation for a well-established principle. I think it would be very useful to keep a list of best-explanations for why the established principle is the established principle, for quick reference (and when marking things as duplicates). Here are some of the common issues and best responses (in my opinion). I hope that this question will open discussions about what other common duplicate questions are, and what the best answers are.
- Galaxies are moving away from each other because of the expansion of space (and not just because they don't like each other).
- Galaxy rotation curves and 'missing mass' are Dark Matter (and not because MOND / dyson spheres / really really really dark normal matter).
- The speed of light is really a constant, from all reference frames (even if you attach rockets to your rockets, in opposite direction of other rockets attached to the aether)
- You cannot measure both position and momentum exactly (no matter how good you are at tennis).
- The double slit experiment cannot be explained by classical physics (despite whatever interesting thing happening on your fishing trip last weekend).