> dark matter and dark energy have always been considered placeholders of sorts It's not a placeholder. We can measure how fast galaxies rotate, run some simul

> dark matter and dark energy have always been considered placeholders of sorts It's not a placeholder. We can measure how fast galaxies rotate, run some simulations with the current law of physics and calculate how much mass they have. Also etimate how much mass we can see. They are different values! Dark matter is the hypotesys that thae different is caused by some mass instead of an error in the laws, or the measurements or something. There are a few hypotesis about what the dark matter actualy is, and they are very different and nonegives a good answer. We can compare the missing mass of calculated with the rotation and other calculations like gravitational lensing and backgroung radiation. IIRC all the these different methods to calculate the ammount of dark matter sorta agree, so people working in the area are quite convincing this is the solution to the problem. > "we don’t know what’s happening here" That's more general. It may mean that equations are wrong. Some people proposed corrections to the equations, but all the proposal so far don't solve the problem. (For example, MOND needs use a new constant a0, but to get the correct result in each galaxy we need a different vaule of a0 instead of a single one for all the universe.)